Foreign particle inspection apparatus and method with front and back illumination

A method of inspecting a phase shift reticle comprising a transparent or translucent substrate, a circuit pattern of an opaque film formed on the front surface of the substrate and a pattern of a transparent or translucent film formed on the front surface of the substrate comprises: obliquely projecting a front illuminating light beam on the front surface of the substrate by a front illuminating system; concentrating scattered light scattered by the surface of the substrate and the surfaces of the patterns, obliquely projecting a back illuminating light beam on the back surface of the substrate by a back illuminating system, concentrating transmitted-and-diffracted light transmitted and diffracted by the substrate and the patterns; intercepting the scattered light scattered by the patterns and the transmitted-and-diffracted light transmitted and diffracted by the patterns with spatial filters disposed on Fourier transform planes, focusing the scattered light and the transmitted-and-diffracted light transmitted by the spatial filters on detectors; and comparing detection signals provided by the detectors to see if there are any foreign particles on the phase shift reticle.

BACKGROUND OF THE INVENTION 
The present invention relates to a method of inspecting a reticle or a 
photomask (hereinafter referred to inclusively as "reticle") provided with 
a circuit pattern and a phase shifter formed of a light-transmissive film 
for defects, such as foreign particles adhering to the reticle, and, more 
particularly, to a method of inspecting a reticle provided with phase 
shifter for defects, such as foreign particles, having sizes on the 
submicron order before printing the reticle on a wafer, and a reticle 
inspecting apparatus for carrying out the same reticle inspecting method. 
When fabricating LSI chips or printed wiring boards, a reticle having a 
circuit pattern is inspected for defects before printing the reticle on 
wafers by a photographic process. If the reticle has minute foreign 
particles having sizes on the submicron order thereon, the reticle cannot 
be correctly printed on wafers and, consequently, LSI chips fabricated by 
using such wafers become defective. Problems attributable to the minute 
foreign particles adhering to the reticle have become more and more 
significant with the recent progressive increase in the degree of 
integration of LSIs and the existence of even foreign particles having 
sizes on the submicron order on the reticle is not permissible. 
The inspection of the reticle for foreign particles before printing the 
reticle on a wafer is indispensable to preventing defective reticle 
printing, and various techniques for inspecting the reticle for foreign 
particles have been proposed. A prevalent method of inspecting a reticle 
for foreign particles, which is employed widely because of its capability 
of quick and highly sensitive inspection, irradiates the reticle obliquely 
with a light beam having a high directivity, such as a laser beam, and 
detects scattered light scattered by foreign particles. However, since the 
light beam is diffracted at edges of the pattern of the reticle, the 
diffracted light and the scattered light scattered by foreign particles 
must be discriminated from each other. Various technical means for 
discriminating between the diffracted light and the scattered light have 
been proposed. 
A first previously proposed technical means is an inspecting apparatus 
disclosed in, for example, Japanese Patent Laid-open (Kokai) No. 
54-101390. This inspecting apparatus comprises a laser that emits a 
linearly polarized laser beam, an irradiating means for irradiating a 
circuit pattern obliquely with the linearly polarized laser beam so that 
the linearly polarized laser beam fall on the circuit pattern at a given 
incidence angle, and an oblique focusing optical system including a 
polarizing plate and lenses. When the circuit pattern is irradiated with 
the linearly polarized laser beam, the diffracted light diffracted by the 
circuit pattern and the scattered light scattered by foreign particles 
differ from each other in the plane of polarization, i.e., the plane of 
vibration, therefore, only the light scattered by the foreign particles 
can be detected. 
A second previously proposed technical means is an inspecting apparatus 
disclosed in, for example, Japanese Patent Laid-open (Kokai) No. 59-65428, 
1-117024 or 1-153943. This inspecting apparatus comprises a scanning means 
for scanning an object with a laser beam obliquely projected on the 
object, a first lens disposed above the object to condense scattered laser 
light so that the point of irradiation of the laser beam coincides 
substantially with the condensing point, a filter plate disposed on the 
Fourier transform image plane of the first lens to filter regular 
diffracted light diffracted by the circuit pattern of the object, a second 
lens for the inverse Fourier transform of the scattered light scattered by 
the foreign particles and transmitted through the screen plate, a slit 
plate disposed at the focal point of the second lens to screen scattered 
light from portions of the object other than a portion corresponding to 
the point of irradiation with the laser beam, and a light receiving device 
for receiving the scattered light scattered by the foreign particles and 
passed through the slit of the slit plate. This inspecting apparatus is 
based on a fact that generally the elements of a circuit pattern are 
extended in a single direction or in several directions, and filters the 
diffracted light diffracted by the elements of the circuit pattern 
extending in a specified direction by a spatial filter disposed on the 
Fourier transform image plane to detect only the scattered light scattered 
by the foreign particles. 
A third previously proposed technical means is an arrangement disclosed in, 
for example, Japanese Patent Laid-open (Kokai) No. 58-62543. This 
arrangement is based on a fact that diffracted light diffracted by the 
edges of a circuit pattern is directional light while scattered light 
scattered by foreign particles is not directional and identifies foreign 
particles on the basis of the logical product of the outputs of a 
plurality of obliquely arranged detectors. 
A fourth previously proposed technical means is an arrangement disclosed 
in, for example, Japanese Patent Laid-open (Kokai) No. 60-154634 or 
60-154635. This arrangement is based on a fact that diffracted light 
diffracted by the edges of a circuit pattern converges only along a 
specific direction while scattered light scattered by foreign particles is 
scattered in all directions and identifies foreign particles from the 
outputs of a plurality of detectors. 
Apparatuses and methods relating to the inspection of objects for minute 
foreign particles, such as a schlieren method, a phase-contrast microscope 
and a technique relating to a diffraction image having a finite size are 
disclosed in, for example, Hiroshi Kubota, "Oyou Kogaku", Iwanami Zensho, 
pp. 129-136. 
When an array type detector, such as a one-dimensional solid-state imaging 
device provided with an array of solid-state image sensors, an output 
signal representing a foreign particle is distributed to a plurality of 
pixels if the foreign particle corresponds to a plurality of elements of 
the detector and, consequently, the output of the detector is reduced and 
therefore there is the possibility that the detector fails in detecting 
the foreign particle. An invention made to obviate such a possibility, 
disclosed in Japanese Patent Laid-open (Kokai) No. 61-104242 disposes an 
array type detector at an angle to the direction of scanning operation of 
an inspection table. Other inventions made for the same purpose, disclosed 
in Japanese Patent Laid-open (Kokai) Nos. 61-104244 and 61-104659 employ 
an array type detector having a unique shape and provided with elements 
arranged in a unique arrangement. 
Irregular illumination and the variation of illumination affects adversely 
to the repeatability and accuracy of detection. An invention disclose in 
Japanese Patent Laid-open (Kokai) No. 60-038827 calibrates the intensity 
of scattered light automatically by using a standard sample having a known 
characteristics. 
Japanese Patent laid-open (Kokai) No. 56-132549 discloses an invention for 
obviating misidentifying a large amount of scattered light scattered by a 
comparatively large foreign particle as scattered light scattered by a 
plurality of comparatively small foreign particles. 
As mentioned above, failure in finding foreign particles which affect 
adversely to the quality of LSI chips has become a significant problem 
with the reduction of the size of foreign particles to be detected. The 
first previously proposed technical means, for example, the invention 
disclosed in Japanese Patent Laid-open (Kokai) No. 54-101390, is unable to 
detect minute foreign particles because the difference between the plane 
of polarization of the scattered light scattered by minute foreign 
particles and the plane of polarization of the diffracted light diffracted 
by the edges of the circuit pattern is small. 
The second previously proposed technical means, for example, the inventions 
disclosed in Japanese Patent Laid-open (Kokai) Nos. 59-65428, 1-117024 and 
1-153943, detect only the scattered light scattered by foreign particles 
by separating the scattered light scattered by foreign particles from the 
diffracted light diffracted by the circuit pattern with the filter plate 
and the slit plate. Although these inventions uses a detecting mechanism 
for detecting foreign particles by a simple binary method having a simple 
configuration to their advantage, the diffracted light diffracted by the 
intersection of the elements of the circuit pattern does not travel 
unidirectionally like the diffracted light diffracted by the straight edge 
of the circuit pattern, and hence the spatial filter is unable to filter 
the diffracted light diffracted by the intersection of the elements of the 
circuit pattern perfectly. Furthermore, since the diffracted light 
diffracted by a minute circuit pattern on the micron order for a LSI 
having a very high degree of integration is analogous in behavior to the 
scattered light scattered by foreign particles and hence it is practically 
difficult to discriminate between the circuit pattern and foreign 
particles by a simple binary method. 
The apparatus proposed as the third previously proposed technical means in, 
for example, Japanese Patent Laid-open (Kokai) No. 58-62543) and the those 
proposed as the fourth previously proposed technical means in, for 
example, Japanese Patent Laid-open (Kokai) Nos. 60-154634 and 60-154635 
have difficulty in employing an optical system having a sufficiently high 
condensing ability because of their configurations and hence it is 
practically difficult for these apparatuses to detect faint scattered 
light scattered by foreign particles. 
The apparatuses disclosed as the fifth previously proposed technical means 
in, for example, Japanese Patent Laid-open (Kokai) Nos. 61-104242 and 
61-104244 need a special detector and a special optical system, which are 
costly. 
The apparatus disclosed as the sixth previously proposed technical means 
in, for example, Japanese Patent Laid-open (Kokai) No. 60-038827 has 
drawbacks in application to an array type detector suitable for quick 
detection and in structural accuracy for detecting minute foreign 
particles. 
The apparatus disclosed as the seventh previously proposed technical means 
in, for example, Japanese Patent Laid-open (Kokai) No. 56-132544 detects 
only a single point on a large foreign particle and hence the apparatus is 
unable to recognize the shape of an elongate foreign particle accurately. 
A reticle recently developed to improve resolution in transferring a 
circuit pattern formed on the reticle is provided with a transparent or 
translucent thin film, which is called a phase shift film or a phase 
shifter, having a thickness equal to odd times half the wavelength of the 
light used for exposure and formed so as to cover spaces between the 
elements of the circuit pattern. Although this thin film is transparent or 
translucent, the thickness of this thin film is several times the 
thickness on the order of 0.1 .mu.m of the circuit pattern. Consequently, 
the intensity of the diffracted light diffracted by the edges of the thin 
film is several to several tens times the intensity of the diffracted 
light diffracted by the edges of the circuit pattern, which reducing the 
foreign particle detecting sensitivity significantly. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a method 
of inspecting a reticle, such as a reticle fabricated by forming a circuit 
pattern and a phase shift film for improving transfer resolution on a 
transparent or translucent substrate for defects, such as minute foreign 
particles having sizes on the submicron order adhering to the circuit 
pattern, capable of separating the defects from the circuit pattern and of 
stably detecting the defects, and an apparatus for carrying out the 
method. 
Another object of the present invention is to provide a projection exposure 
method capable of eliminating defects, such as minute foreign particles, 
from a mask, such as a reticle provided with a phase-shift film 
(hereinafter referred to as "phase shift reticle") and of projecting a 
circuit pattern formed on the mask on a substrate, such as a wafer, by 
using an image-reducing projection exposure apparatus for exposure, and an 
apparatus for carrying out the projection exposure method. 
The present invention has been made by introducing improvements into the 
invention proposed in U.S. patent application Ser. No. 08/192,036 related 
with U.S. patent application Ser. No. 07/902,819 or the invention proposed 
in Korean Pat. Application No. 3209/1994 related with Korean Pat. 
Application No. 11092/1992. 
With the foregoing object in view, the present invention provides a reticle 
inspecting apparatus for inspecting a reticle fabricated by forming a 
circuit pattern on a substrate, for defects, such as foreign particles 
adhering to the reticle, comprising: a stage unit for supporting the 
substrate, capable of being optionally moved in directions along an 
X-axis, a Y-axis and a Z-axis; an illuminating system having a first 
illuminating system and a second illuminating system individually provided 
with light sources that emit light beams of approximately 780 nm in 
wavelength, disposed opposite to each other on the side of the front 
surface of the substrate provided with the circuit pattern so as to 
illuminate the substrate with light beams that travel obliquely to the 
surface of the substrate, either the first or the second illuminating 
system that can project the light beam on the surface of the substrate so 
that the light beam is not intercepted by a pellicle holding frame being 
used; an illuminating system disposed on the opposite side of the reticle 
with respect to the former illuminating system, having a third 
illuminating system corresponding to the first illuminating system and a 
fourth illuminating system corresponding to the second illuminating 
system, individually provided with light sources that emit light beams of 
approximately 488 nm in wavelength, the third illuminating system being 
used when the first illuminating system is used or the fourth illuminating 
system being used when the second illuminating system is used; a focusing 
optical system having a high numerical aperture (NA) of 0.4 or above, 
disposed on the side of the front surface of the substrate provided with 
the circuit pattern, that does not concentrate directly reflected light 
and directly transmitted light, concentrates scattered light scattered by 
a portion of the circuit pattern and diffracted light diffracted by the 
same portion of the circuit pattern, separates the scattered light and the 
diffracted light according to the direction of illumination and 
wavelength, intercepts the diffracted light diffracted by straight 
portions of the circuit pattern with spatial filters disposed on the 
Fourier transform planes of the separated light, and focuses the light 
from the illuminated area on detectors; and a signal processing system 
that provides the defect data of foreign matters and the like on the 
circuit pattern by operating binary data obtained by binarizing the 
outputs of the detectors by a binarizing circuit set for a threshold, and 
signals representing the logical product of the binary data. 
The present invention provides a method of detecting defects, such as 
foreign particles adhering to a substrate, comprising: projecting a front 
illuminating light beam obliquely on the front surface of a transparent or 
translucent substrate provided with a masking pattern and a pattern of a 
transparent or translucent film by a front illuminating system, 
concentrating scattered light scattered by the front surface of substrate 
and the pattern formed on the substrate, projecting a back illuminating 
light beam obliquely on the back surface of the substrate, concentrating 
transmitted-and-diffracted light transmitted and diffracted by the 
substrate and the pattern formed on the substrate, intercepting the 
scattered light scattered by the pattern and the 
transmitted-and-diffracted light transmitted and diffracted by the pattern 
with the spatial filters disposed on the Fourier transform planes to form 
images on detectors, and comparing the output of the detector representing 
the scattered light and that of the detector representing the 
transmitted-and-diffracted light to detect defects, such as foreign 
particles adhering to the substrate. 
The present invention provides a method of detecting defects, such as 
foreign particles adhering to a substrate, comprising: projecting a front 
illuminating light beam obliquely on a transparent or translucent 
substrate provided with a pattern of an opaque film and a pattern of a 
transparent or translucent film by a front illuminating system, 
concentrating scattered light scattered by the surface of the substrate 
and the pattern formed on the substrate, projecting a back illuminating 
light beam obliquely on the back surface of the substrate by a back 
illuminating system, concentrating transmitted-and-diffracted light 
transmitted and diffracted by the substrate and the pattern formed on the 
substrate, separating the concentrated scattered light and the 
concentrated transmitted-and-diffracted light, intercepting the scattered 
light scattered by the pattern and the transmitted-and-diffracted light 
transmitted and diffracted by the pattern with spatial filters disposed on 
Fourier transform planes to focus images on a first detector and a second 
detector, respectively, and comparing the outputs of the first detector 
and the second detector to detect defects, such as foreign particles 
adhering to the substrate. 
The method of detecting defects in accordance with the present invention is 
characterized in that the substrate is a component of a phase shift 
reticle. The method of detecting defects in accordance with the present 
invention is characterized in concentrating the scattered light and the 
transmitted-and-diffracted light by a focusing optical system disposed on 
the side of the front surface of the substrate with its optical axis 
substantially perpendicular to the surface of the substrate. The method of 
detecting defects in accordance with the present invention is 
characterized in illuminating an area in the front surface of the 
substrate with the front illuminating light beam along a plurality of 
directions and illuminating an area in the back surface of the substrate 
with the back illuminating light beam along a plurality of directions. The 
method of detecting defects in accordance with the present invention is 
characterized in using the front illuminating light beam and the back 
illuminating light beam alternately. The method of detecting defects in 
accordance with the present invention is characterized in that the front 
illuminating light beam and the back illuminating light beam are different 
in wavelength from each other, and the wavelength of the front 
illuminating light beam is longer than that of the back illuminating light 
beam. The method of detecting defects in accordance with the present 
invention is characterized in that the wavelength of the front 
illuminating light beam is in the range of 600 to 800 nm and the 
wavelength of the back illuminating light beam is in the range of 450 to 
550 nm. 
The present invention provides a projection exposure method comprising: a 
defect detecting step in which the front surface of the transparent or 
translucent substrate of a phase shift reticle, which is fabricated by 
forming a circuit pattern of an opaque thin film and a pattern of a 
transparent or translucent film on the substrate and attaching covering 
means formed by attaching transparent films to frames, respectively, to 
the opposite surfaces of the substrate to prevent the adhesion of foreign 
matters to the surfaces of the substrate, is illuminated through the 
transparent film of the covering means with a front illuminating light 
beam projected obliquely on the front surface of the substrate by a front 
illuminating system, scattered light scattered by the front surface of the 
substrate and by the pattern formed on the substrate is received through 
the transparent film of the covering means and concentrated, the back 
surface of the substrate is illuminated through the transparent film of 
the covering means with a back illuminating light beam projected obliquely 
on the back surface of the substrate through the transparent film of the 
covering means by a back illuminating system, transmitted-and-diffracted 
light transmitted and diffracted by the substrate and the pattern formed 
on the substrate is received through the transparent film of the covering 
means and concentrated, the concentrated scattered light and the 
concentrated transmitted-and-diffracted light are separated, the scattered 
light scattered by the pattern and the transmitted-and-diffracted light 
transmitted and diffracted by the pattern are intercepted with spatial 
filters disposed on Fourier transform planes, the light transmitted by the 
spatial filters is focused on a first detector and a second detector, and 
the outputs of the first detector and the second detector are compared to 
detect defects, such as foreign particles adhering to the substrate; a 
conveying step in which the phase shift reticle is conveyed, when it is 
confirmed in the defect detecting step that no defect is found on the 
substrate, to an exposure position in a projection exposure apparatus; and 
an exposure step in which a substrate to be processed is exposed to 
exposure light transmitted the phase shift reticle and focused by a 
focusing optical system and the phase of the light transmitted by the 
adjacent transparent or translucent film is shifted to prevent 
interference to form an image of the circuit pattern of the opaque film on 
the substrate. 
The present invention provides a projection exposure method comprising: a 
first defect detecting step in which the front surface of the transparent 
or translucent substrate of a phase shift reticle, provided with a circuit 
pattern of an opaque film and a pattern of a transparent or translucent 
film is illuminated with a front illuminating light beam projected 
obliquely on the front surface of the substrate by a front illuminating 
system, scattered light scattered by the front surface of the substrate 
and the surfaces of the pattern formed on the substrate is concentrated, 
the back surface of the substrate is illuminated with a back illuminating 
light beam projected obliquely on the back surface of the substrate, 
transmitted-and-diffracted light transmitted and diffracted by the 
substrate and the pattern formed on the substrate is concentrated, the 
concentrated scattered light and the concentrated 
transmitted-and-diffracted light are separated, the scattered light 
scattered by the pattern and the transmitted-and-diffracted light 
transmitted and diffracted by the pattern are intercepted with spatial 
filters disposed on Fourier transform planes, the light transmitted by the 
spatial filters is focused on a first detector and a second detector, the 
outputs of the first detector and the second detector are compared to 
detect defects, such as foreign particles adhering to the substrate; a 
cover attaching step in which, when it is confirmed in the first defect 
detecting step that no defect is found in the phase shift reticle, 
covering means formed by attaching transparent films to frames, 
respectively, for preventing preventing the adhesion of foreign matters to 
the phase shift reticle are attached to the opposite surfaces of the phase 
shift reticle; a second second defect detecting step in which the front 
surface of the substrate is illuminated through the transparent film of 
the covering means with a front illuminating light beam projected 
obliquely on the front surface of the substrate by the front illuminating 
system, scattered light scattered by the front surface and the surfaces of 
the pattern formed on the substrate is concentrated, the back surface of 
the substrate is illuminated through the transparent film of the covering 
means with a back illuminating light beam projected obliquely on the back 
surface of the substrate by the back illuminating system, 
transmitted-and-diffracted light transmitted and diffracted by the 
substrate and the pattern formed on the substrate is received through the 
transparent film of the covering means and concentrated, the concentrated 
scattered light and the concentrated transmitted-and-diffracted light are 
separated, the scattered light scattered by the pattern and the 
transmitted-and-diffracted light transmitted and diffracted by the pattern 
are intercepted with spatial filters disposed on Fourier transform planes, 
the scattered light and the transmitted-and-diffracted light transmitted 
by the spatial filters are focused on the first detector and the second 
detector, respectively, and the outputs of the first detector and the 
second detector are compared to detect defects, such as foreign particles 
adhering to the substrate; a conveying step in which the phase shift 
reticle provided with the covering means is conveyed, when it is confirmed 
in the second defect detecting step that no defect is found on the 
substrate, to an exposure position in an projection exposure apparatus; 
and an exposure step in which a substrate to be processed is exposed to 
exposure light transmitted by the phase shift reticle and focused by a 
focusing system and the phase of the light transmitted by the adjacent 
transparent or translucent film is shifted to prevent interference to form 
an image of the circuit pattern of the opaque film on the substrate. 
The present invention provides a defect detecting apparatus comprising: a 
front illuminating system that illuminates obliquely the front surface of 
a transparent of translucent substrate provided with a pattern of an 
opaque film and a pattern of a transparent or translucent film with a 
front illuminating light beam; a back illuminating system that illuminates 
obliquely the back surface of the substrate with a back illuminating light 
beam; a focusing optical system that concentrates and focuses scattered 
light, i.e., the front illuminating light beam emitted by the front 
illuminating system and scattered by the front surface of the substrate 
and the surface of the pattern and transmitted, and diffracted light, 
i.e., the back illuminating light beam emitted by the back illuminating 
system, and transmitted and diffracted by the substrate and the pattern 
formed on the substrate; spatial filters disposed on Fourier transform 
image planes to intercept the scattered light scattered by the pattern and 
the transmitted-and-diffracted light transmitted and diffracted by the 
pattern; detectors that receive the concentrated scattered light the 
concentrated transmitted-and-diffracted light transmitted by the spatial 
filters and provide output signals corresponding to the receive light; and 
detecting means that compares the output signal of the detector 
representing the scattered light and the output signal of the detector 
representing the transmitted-and-diffracted light to detect defects, such 
as foreign particles adhering to the substrate. 
The present invention provides a defect detecting apparatus comprising: a 
front illuminating system that illuminates obliquely the front surface of 
a transparent or translucent substrate provided with a pattern of an 
opaque film and a pattern of a transparent or translucent film with a 
front illuminating light beam; a back illuminating system that illuminates 
obliquely the back surface of the substrate with a back illuminating light 
beam; a focusing optical system that concentrates and focuses scattered 
light, i.e., the front illuminating light beam emitted by the front 
illuminating system and scattered by the surface of the substrate and the 
surface of the pattern formed on the substrate, and 
transmitted-and-diffracted light, i.e., the back illuminating light beam 
emitted by the back illuminating system and transmitted and diffracted by 
the substrate and the pattern formed on the substrate; a separating 
optical system that separates the scattered light and the 
transmitted-and-diffracted light; a first spatial filter and a second 
spatial filter disposed on Fourier transform planes to intercept the 
scattered light scattered by the pattern and the 
transmitted-and-diffracted light transmitted and diffracted by the 
pattern; a first detector and a second detector that receives the 
scattered light and the transmitted-and-diffracted light focused by the 
focusing optical system, respectively, and provides output signals; and 
detecting means that compares the output signal of the first detector 
representing the scattered light and the output signal of the second 
detector representing the transmitted-and-diffracted light to detect 
defects, such as foreign particles adhering to the substrate. 
The present invention provides a projection exposure apparatus comprising: 
a defect detecting means that illuminates obliquely the front surface of 
the transparent or translucent substrate of a phase shift reticle 
fabricated by forming a circuit pattern of an opaque film and a pattern of 
a transparent or translucent film on the front surface of the substrate 
and provided on the front and the back surfaces of the substrate with 
covering means formed by attaching transparent films respectively to 
frames with a front illuminating light beam emitted by a front 
illuminating optical system through the transparent film of the covering 
means, concentrates scattered light scattered by the front surface of the 
substrate and the surface of the pattern, illuminates obliquely the back 
surface of the substrate with a back illuminating light beam emitted by a 
back illuminating system through the transparent film of the covering 
means, concentrates transmitted-and-diffracted light transmitted and 
diffracted by the substrate and the pattern and traveling through the 
transparent film of the covering means, separates the concentrated 
scattered light and the concentrated transmitted-and-diffracted light, 
intercepting the scattered light scattered by the pattern and the 
transmitted-and-diffracted light transmitted and diffracted by the pattern 
with spatial filters disposed on Fourier transform planes, focuses the 
scattered light and the transmitted-and-diffracted light respectively on a 
first detector and a second detector, and compares the outputs of the 
first detector and the second detector to detect defects, such as foreign 
particles adhering to the substrate; conveying means that conveys the 
phase shift reticle provided with the covering means, when it is confirmed 
by the defect detecting means that no defect is found on the substrate, to 
an exposure position; and a projection exposure means that exposes a 
substrate to be processed to exposure light transmitted by the phase shift 
reticle and focused by a focusing optical system and shifts the phase of 
the light transmitted by the adjacent transparent or translucent film to 
prevent interference to form an image of the circuit pattern of the opaque 
film on the substrate. 
The present invention provides a projection exposure apparatus comprising: 
a defect detecting means that illuminates obliquely the front surface of 
the transparent or translucent substrate of a phase shift reticle 
fabricated by forming a circuit pattern of an opaque film and a pattern of 
a transparent or translucent film on the front surface of the substrate 
with a front illuminating light beam emitted by a front illuminating 
system, concentrating scattered light scattered by the front surface of 
the substrate and the surface of the pattern, illuminates obliquely the 
back surface of the substrate by a back illuminating system, concentrates 
transmitted-and-diffracted light transmitted and diffracted by the 
substrate and the pattern, separates the scattered light and the 
transmitted-and-diffracted light, intercepts the scattered light scattered 
by the pattern and the transmitted-and-diffracted light transmitted and 
diffracted by the pattern with spatial filters disposed on Fourier 
transform planes, focuses the scattered light and the 
transmitted-and-diffracted light transmitted by the spatial filters on a 
first detector and a second detector, and compares the outputs of the 
first detector and the second detector to detect defects, such as foreign 
particles adhering to the substrate; a conveying means that conveys the 
phase shift reticle, when it is confirmed by the defect detecting means 
that no defect is found on the substrate, to an exposure position; and a 
projection exposure means that illuminates a substrate to be processed 
with exposure light transmitted by the phase shift reticle located at the 
exposure position and focused by a focusing optical system and shifts the 
phase of the light transmitted by the adjacent transparent or translucent 
film to prevent interference to form an image of the circuit pattern of 
the opaque film on the substrate. 
The present invention provides a projection exposure apparatus comprising: 
a defect detecting means that illuminates obliquely the front surface of 
the transparent or translucent substrate of a phase shift reticle 
fabricated by forming a circuit pattern of an opaque film and a pattern of 
a transparent or translucent film on the front surface of the substrate 
with a front illuminating light beam by a front illuminating system, 
concentrates scattered light scattered by the front surface of the 
substrate and the surface of the pattern formed on the substrate, 
illuminates obliquely the back surface of the substrate with a back 
illuminating light beam by a back illuminating system, concentrates 
transmitted-and-diffracted light transmitted and diffracted by the 
substrate and the pattern formed on the substrate, separates the scattered 
light and the transmitted-and-diffracted light, intercepts the scattered 
light scattered by the pattern and the transmitted-and-diffracted light 
transmitted and diffracted by the pattern with spatial filters disposed on 
Fourier transform planes, focuses the scattered light and the 
transmitted-and-diffracted light transmitted by the spatial filters on a 
first detector and a second detector, respectively, and compares the 
outputs of the first detector and the second detector to detect defects, 
such as foreign particles adhering to the substrate; a conveying means 
that conveys the phase shift reticle, when it is confirmed by the defect 
detecting means that no defect is found on the substrate, to an exposure 
position; and a projection exposure means that exposes a substrate to be 
processed to exposure light transmitted by the phase shift reticle located 
at the exposure position and focused by a focusing optical system and 
shifts the phase of the light transmitted by the adjacent transparent or 
translucent film to prevent interference to form an image of the circuit 
pattern of the opaque film on the substrate. 
The present invention provides a projection exposure apparatus comprising: 
a defect detecting means that illuminates obliquely the front surface of a 
transparent or translucent substrate of a phase shift reticle fabricated 
by forming a circuit pattern of an opaque film and a pattern of a 
transparent or translucent film with a front illuminating light beam 
emitted by a front illuminating system before and after attaching a 
covering means to the phase shift reticle, concentrates scattered light 
scattered by the front surface of the substrate and the surface of the 
pattern formed on the substrate, illuminates obliquely the back surface of 
the substrate with a back illuminating light beam emitted by a back 
illuminating system, concentrates transmitted-and-diffracted light 
transmitted and diffracted by the substrate and the pattern formed on the 
substrate, separates the concentrated scattered light and the concentrated 
transmitted-and-diffracted light, intercepts the scattered light scattered 
by the pattern and the transmitted-and-diffracted light transmitted and 
diffracted by the pattern with spatial filters disposed on Fourier 
transform planes, focuses the scattered light and the 
transmitted-and-diffracted light transmitted by the spatial filters on a 
first detector and a second detector, compares the outputs of the first 
detector and the second detector to detect defects, such as foreign 
particles adhering to the substrate; a conveying means that conveys the 
phase shift reticle provided with the covering means, when it is confirmed 
that no defect is found on the substrate by the defect detecting means, to 
an exposure position; and a projection exposure means that illuminates a 
substrate to be processed with exposure light transmitted by the phase 
shift reticle and focused by a focusing optical system and shifts the 
phase of the light transmitted by the adjacent transparent or translucent 
film to prevent interference to form and image of the circuit pattern of 
the opaque film on the substrate. 
Incidentally, in an exposure process employing a reticle or the like for 
fabricating LSI circuits or printed wiring boards, a circuit pattern 
formed on the reticle or the like is inspected before printing the circuit 
pattern on wafers. If the circuit pattern has minute foreign particles 
having sizes, for example, even on the micron order thereon, the circuit 
pattern cannot correctly be printed on wafers and, consequently, all the 
LSI circuits fabricated by using such wafers become defective. Problems 
attributable to the minute foreign particles adhering to the reticle have 
become more and more significant with the recent progressive increase in 
the degree of integration of LSI circuits and the existence of even 
foreign particles having sizes on the submicron order on the reticle is 
not permissible. 
Increase in frequency of missing finding foreign particles and the like 
that affect adversely to the fabrication of LSI circuits, which is liable 
to increase as the size of foreign particles to be found decreases, must 
be avoided. Recently, a reticle provided with a circuit pattern, and a 
pattern of a phase shift film or a transparent or translucent film called 
a phase shifter filling up spaces in the circuit pattern and having a 
thickness substantially equal to an odd multiple of half the wavelength of 
exposure light has been developed print a circuit pattern of a metal thin 
film, such as a chromium thin film, formed on the reticle in an improved 
resolution. The thickness of this transparent or translucent phase shift 
pattern is several times that of the circuit pattern on the order of 0.1 
.mu.m. Therefore, the intensity of diffracted light diffracted by the 
edges of the phase shift pattern is several to tens times that of 
diffracted light diffracted by the edges of the circuit pattern, and such 
a high intensity of diffracted light reduces the sensitivity of detection 
of foreign particles remarkably. 
(1) Detection of Front/Back Surface Logical Product, Processing Circuit and 
Switching of Illumination 
As stated above, it is difficult to discriminate, by the conventional 
techniques, between the circuit pattern of reticles including the phase 
shift reticle for fabricating DRAMs of, for example, 64M or above and 
foreign particles adhering to the reticles and to detect only the foreign 
particles. 
The present invention solves the foregoing problems on the basis of an 
experimental fact discovered by the inventors of the present invention 
that the intensity of scattered light scattered by the circuit pattern of 
a reticle is dependent on the incidence angle of the illuminating light 
beam, which will be described with reference to FIG. 10. 
In FIG. 10, points 701 and 702 indicate detection signals corresponding to 
the intensity of scattered light scattered by a defect 70, such as a 
minute foreign particle, of 0.5 .mu.m or below in size, points 864, 874, 
865, 875, 866, 876, 867 and 877 indicate detection signals corresponding 
to the intensities of scattered light scattered by all the corners 82 of 
0-degree, 45-degree and 90-degree edges of a circuit pattern, and points 
701, 861, 862, 863, 864, 865, 866 and 867 indicate detection signals 
corresponding to the intensities of scattered light scattered by a minute 
circuit pattern of a size 84 on the order of 0.5 .mu.m. The points 701, 
861, 862, 863, 864, 865, 866 and 867 indicate the detection signals 
provided when the illuminating light beam projected by a first 
illuminating system 2 (or 3) of FIG. 1 and scattered by the circuit 
pattern is detected. The points 702, 871, 872, 873, 874, 875, 876 and 877 
indicate the detection signals provided when the illuminating light beam 
projected by a second illuminating system 20 (or 30) of FIG. 1 and 
scattered by the circuit pattern is detected. For example, detection 
signals 861.rarw..fwdarw.871 are those provided when the illuminating 
light beam projected by the first illuminating system 2 (or 3) of FIG. 1 
and scattered by a portion of the minute circuit pattern is detected and 
when the illuminating light beam projected by the second illuminating 
system 20 (or 30) of FIG. 1 and scattered by the same portion of the 
minute circuit pattern is detected, respectively. As is obvious from FIG. 
10, the value of the detection signal provided upon the detection of the 
defect 70, such as a foreign particle, is less dependent on the direction 
of projection of the illuminating light beam than the value of the 
detection signal provided upon the detection of the circuit pattern. In 
FIG. 10, a dotted line indicate a threshold for detection signals. 
As is obvious from FIG. 10, the value of the detection signal provided when 
a portion of a minute circuit pattern is detect is greatly dependent on 
the direction of projection of the illuminating light beam and, when a 
portion of the surface of a reticle 6 is illuminated obliquely with two 
illuminating light beams traveling respectively in opposite directions, 
either of the detection signals provided upon the detection of the two 
illuminating light beams scattered by the illuminated portion is always 
smaller than the detection signal provided upon the detection of the 
scattered light scattered by the foreign particle of a size on the 
submicron order as indicated by solid circles. 
Therefore, when a portion of the surface of the reticle 6 is illuminated 
with both the illuminating light beams traveling respectively in opposite 
directions as shown in FIG. 1, the detection signal indicating a particle 
or a circuit pattern is merely the sum of a detection signal provided upon 
the detection of one of the illuminating light beams scattered by the 
particle or the circuit pattern and a detection signal provided upon the 
detection of the other illuminating light beam scattered by the same, and 
it is difficult to binarize the detection signal by using a threshold. 
However, when one of the scattered illuminating light beams and the other 
scattered illuminating light beam are detected respectively by two 
detectors, and the detection signals provided by the two detectors are 
binarized by using the threshold 91 respectively by two binarizing 
circuits, both the detection signals for the foreign particle or the like 
are "1", and either of the detection signals for the circuit pattern is 
"1" or both the detection signals for the circuit pattern are "0". 
Accordingly, the defect 70, such as a foreign particle of a size on the 
submicron order can be discriminated from the circuit pattern on the basis 
of the logical product of the binary outputs of the binarizing circuits. 
(2) Back Illuminating System, Coherence Length, Balance of Amount of Light, 
Correction of Optical Path Length, Correction of Illuminated Position 
As stated above, it is difficult for the conventional techniques to 
discriminate between the circuit pattern of a reticle, such as a phase 
shift reticle, to be used for fabricating, for example, a DRAM of 64M or 
greater and foreign particles adhering to the reticle, and to detect only 
the foreign particles. 
The present invention relates to a foreign particle detecting method of a 
back illumination system capable of detecting foreign particles adhering 
to a phase shift reticle without being affected by the phase shifter of 
the phase shift reticle. When inspecting a photomask, such as a reticle, 
having a glass substrate foreign particles, this foreign particle 
detecting method inserts a glass plate having a thickness corresponding to 
the difference between a standard thickness and the thickness of the glass 
substrate between the back surface of the reticle and the optical system 
to adjust the optical path length between the optical system and the front 
surface of the reticle on which a circuit pattern is formed to a fixed 
length regardless of the thickness of the reticle to avoid the variation 
of concentration of the illuminating light beam and the illuminated 
position according to the thickness of the glass substrate. 
(3) Spatial Filter for Observation and Oblique Projection of Laser beam 
In some cases, it is difficult to observe foreign particles by an 
observation optical system using ordinary drop illumination or 
transmission illumination when the foreign particles have a very small 
size on the order of, for example, 0.3 .mu.m. Furthermore, such an object 
cannot be illuminated in a brightness enough for observation by ordinary 
dark field illumination. 
Practically, it is effective to illuminate such an object obliquely with a 
laser beam and to observe scattered light. Although it is desirable to use 
a laser beam for detection for observation, an additional TV camera or the 
like sensitive to the wavelength of the laser beam will be necessary for 
observation if the laser beam is not visible radiation. Therefore, a laser 
illuminating system that emits a laser beam having a wavelength in the 
wavelength range of visible radiation is used specially for observation. 
The use of a laser illuminating system employing a laser diode having an 
oscillation wavelength in the wavelength range of visible radiation will 
enable the system to be formed in a small, simple construction. 
An observation system employing oblique illumination with a laser beam, 
similarly to a detecting system, is able to eliminate scattered light 
scattered by a circuit pattern by a spatial filter. Therefore, an 
observation system capable of incorporating a spatial filter when 
necessary is able to stress scattered light scattered by defects, such as 
foreign particles, which facilitate the identification of the defects. 
(4) Measurement of the Transmittance of a Pellicle 
A pellicle for protecting a circuit pattern formed on a reticle has 
characteristics, including an antireflection characteristic, to suppress 
the reduction of the intensity of transmitted exposure light (in most 
cases, near-ultraviolet light or ultraviolet light). Since the 
characteristics of the reticle is optimized for use in connection with 
exposure light, the characteristics are not necessarily optimum for use in 
connection with illuminating light for inspection and the reticle reduces 
the intensity of the illuminating light for inspection. Since different 
pellicles have different illuminating light reducing ratios, the use of a 
pellicle narrows the allowance of criterion for inspection. 
If a light beam travels perpendicularly through a pellicle (scattered light 
scattered by a sample in this embodiment travels in such a way), the 
difference in illuminating light reducing ratio between different 
pellicles is negligibly small. However, a light beam that travels 
obliquely through a pellicle (an illuminating light projected obliquely on 
a sample in this embodiment travels in such a way), the transmittance of 
the pellicle is an important factor. A method of measuring the 
transmittance of a pellicle which affects the transmission of a light beam 
that travels obliquely through the pellicle will be described hereinafter. 
A detection output correcting method proposed in Japanese Patent Laid-open 
(Kokai) No. 4-151663 measures the transmittance of a pellicle incorporated 
into a reticle for the inspection of every and corrects the detection 
output. This known detection output correcting method projects a laser 
beam perpendicularly to the pellicle, measures the intensity of regularly 
reflected light from a chromium film formed over the surface of the 
substrate of the reticle, and calculates the ratio of the intensity of the 
regularly reflected light to that of the light before falling on the 
pellicle to determine the transmittance of the pellicle. Since the 
measurement is affected by both the reflectivity of the pellicle and that 
of the substrate of the reticle, the reflectivity of the substrate affects 
measuring accuracy. When the illuminating light beam for detection is 
projected obliquely on the reticle as the illuminating light beam is 
projected in the present invention, the laser beam for measurement must be 
projected obliquely so as to fall on the surface of the substrate at an 
incidence angle equal to that at which the illuminating light for 
detection falls on the surface of the substrate, which is different from 
the known detection output correcting method. 
The present invention projects obliquely a light beam having a wavelength 
equal to that of the illuminating light beam for detection on the pellicle 
at an incidence angle equal to that at which the illuminating light beam 
for detection falls on the pellicle, measures the intensity of reflected 
light from the pellicle, and determines the reflectivity of the pellicle 
on the basis of the measured intensity of the reflected light from the 
pellicle and the intensity of the light emitted by the light source. 
Supposing that the internal absorbance of the pellicle is negligible, 
EQU (Intensity of transmitted light)=(Intensity of incident light)-( Intensity 
of reflected light) 
EQU (Transmittance)=(Intensity of transmitted light)/(Intensity of incident 
light) 
Therefore, 
EQU (Transmittance)={(Intensity of incident light)-(Intensity of reflected 
light)}/(Intensity of incident light) 
The detection output is corrected by using the calculated transmittance. 
(5) Switching of Spatial Filter, Polarizing Plate: Detection of Four-Pixel 
Maximum Value and Detection of Logical Sum 
The present invention has been made on the basis of a fact that 
diffracted/scattered light (hereinafter, "diffracted light"), which is 
part of an illuminating light projected obliquely on a photomask, such as 
a reticle, and diffracted by the circuit pattern of the photomask, such as 
a reticle, is concentrated in a specific area on the Fourier transform 
plane of a plane including the circuit pattern, particularly, in the 
linear central area as shown by photographs on the left-hand side of FIGS. 
73(A) to 73(C). White portions in these photographs of FIGS. 73(A) to 
73(C) are different kinds of diffracted light, i.e., part of an 
illuminating light beam projected obliquely on and diffracted by different 
circuit patterns, as observed on the Fourier transform planes of the 
circuit patterns. Although some kinds of diffracted light form different 
diffraction patterns, most kinds of diffracted light are concentrated in 
the central areas of the Fourier transform planes in linear patterns as 
shown in FIGS. 73(A) to 73(C). Therefore, the scattered light scattered by 
a foreign particle and traveling through the light-transmissive area of 
the spatial filter can be detected by intercepting most part of the 
diffracted light diffracted by the circuit pattern with a spatial filter 
having a shading portion having an appropriate width selected from among 
spatial filters having shading portions of different widths, respectively, 
capable of shading the central areas of the corresponding Fourier 
transform planes as shown on the right-hand side of FIGS. 73(A) to 73(C), 
because the scattered light scattered by a foreign particle cannot be 
concentrated in the central area of the Fourier transform plane. 
In a very rare case, a type of a photomask, such as a reticle, does not 
diffract light so that the diffracted light will not be concentrated in 
the central area of the Fourier transform plane as shown in FIG. 73(D). 
For such a reticle, a polarizing filter may be disposed on the Fourier 
transform plane. 
When an operation for the detection and determination of defects, such as 
foreign particles, is executed by an array type detector, such as a CCD, 
for each pixel, and a defect, such as a foreign particle, extends over a 
plurality of pixels, for example, two to four pixels, scattered light from 
the defect is distributed to a plurality of pixels and the magnitude of 
the detection signal of each pixel is 1/2 to 1/4 of that of the same when 
all the scattered light falls thereon, so that the repeatability of 
inspection is deteriorated. To solve such a problem, a four-pixel addition 
method proposed in Japanese Patent Laid-open (Kokai) No. 5-2262 reduces 
each side of the pixel by half (by a factor of four in area), and adds the 
detection signals of the adjacent four pixels electrically to simulate the 
detection signal of a desired pixel. 
If the size of a foreign particle, for example, 0.5 .mu.m, is small as 
compared with the size of a detection pixel, for example, 2 .mu.m.times.2 
.mu.m, the detection signal representing the intensity of scattered light 
from the defect, provided before the four-pixel addition operation and the 
detection signal obtained through the four-pixel addition operation are 
equal to each other, because the four-pixel addition operation provides 
the sum of the detection signals of a plurality of pixels corresponding to 
the defect. In this case, since the smaller the area (size) of each pixel 
of the detector, the smaller is the number (or the area) of corners of the 
circuit pattern corresponding to one pixel, the scattered light from the 
circuit pattern decreases. Therefore, smaller pixels are more preferable 
and enables the detection of defects in a higher detection sensitivity. 
Accordingly, the four-pixel addition method reduces and sacrifices the 
detection sensitivity for the stabilization of detection. Although any 
measures need not be taken if the reduced detection sensitivity is high 
enough, some measures must be taken to make the detection sensitivity 
effective even if process conditions and exposure method are changed. 
This problem may be solved by selectively using either a high-stability 
detection mode including the four-pixel addition operation or a 
high-sensitivity detection mode not including the four-pixel addition 
operation according to required performance. 
The present invention provides means capable of achieving both the purposes 
of the high-stability detection mode and the high-sensitivity detection 
mode by carrying out the detection of foreign particles before and after 
the four-pixel addition operation and providing the logical sum of the 
results of detection in the high-stability detection mode and in the 
high-sensitivity detection mode on the basis of a fact that both the 
high-stability detection mode and the high-sensitivity detection mode can 
be carried out if defects, such as foreign particles, are detected before 
and after the four-pixel addition operation. 
When the two modes are used synchronously, the amount of data to be 
processed before the four-pixel addition operation is four times that of 
data to be processed after the four-pixel addition operation, because one 
piece of data is obtained from four pixels after the four-pixel addition 
operation. However, if only the maximum detection data among those 
provided by the four adjacent pixels before the four-pixel addition 
operation is used, the amount of data is reduced to 1/4 and the amount of 
data to be processed before the four-pixel addition operation and that of 
data to be processed after the four-pixel addition operation are equal to 
each other, which facilitates the operation for obtaining the logical sum. 
(6) Two-Pixel Addition Method 
When an operation for the detection and determination of defects, such as 
foreign particles, is executed by an array type detector, such as a CCD, 
for each pixel, and a defect, such as a foreign particle, extends over a 
plurality of pixels, for example, two to four pixels, scattered light from 
the defect is distributed to a plurality of pixels and the magnitude of 
the detection signal of each pixel is 1/2 to 1/4 of that of the same when 
all the scattered light falls thereon, so that the repeatability of 
inspection is deteriorated. To solve such a problem, the four-pixel 
addition method proposed in Japanese Patent Laid-open (Kokai) No. 5-2262 
reduces each side of the pixel by half (by a factor of four in area), and 
adds the detection signals of the adjacent four pixels electrically to 
simulate the detection signal of a desired pixel. 
Since this known method is intended to prevent the reduction of the 
detection signal indicating the defect extending over a plurality of 
pixels, the number of pixels of each pixel group may be more than four, 
two or three provided that the desired object can be achieved. 
The present invention uses a rectangular pixel that can be realized by 
feeding the stage at a high feed speed as compared with data storing time 
required by the detector for storing data. For example, if the detector 
has pixels each corresponding to an area of 1 .mu.m.times.1 .mu.m on a 
sample, and the stage is moved 2 .mu.m in a data storing time T, each 
pixel corresponds to an area of 1 .mu.m.times.2 .mu.m on the sample. 
In this case, the addition of the output signals of two pixels corresponds 
to the output signal of a desired pixel. Although the effect of the 
two-pixel addition method on preventing the reduction of the detection 
signal indicating the defect extending over four pixels is lower than that 
of the four-pixel addition method, the two-pixel addition method improves 
the inspection speed because the stage feed speed of the two-pixel 
addition method is higher than that of the four-pixel addition method. 
(7) Pellicle/Glass Surface Detecting System 
Since the present invention uses a high-resolution detector, it is 
difficult for the present invention to carryout high-speed defect 
detection, and the present invention is disadvantageous in respect of 
inspection time as compared with the conventional method. When a 
photomask, such as a reticle, is inspected for defects, the back surface 
of the photomask opposite the front surface of the same on which a circuit 
pattern is formed, i.e., the surface of a glass plate not provided with 
any circuit pattern, and the surface of the pellicle must be inspected in 
addition to the inspection of the circuit pattern, which must be inspected 
in a high detection sensitivity. Since those surfaces may be inspected in 
a detection sensitivity far lower than that necessary for inspecting the 
circuit pattern, the application of a high-resolution high-sensitivity 
inspecting method to the inspection of those surfaces wastes time 
uselessly. 
An inspecting method proposed in Japanese Patent Laid-open (Kokai) No. 
4-273008 employs, instead of increasing the speed, a detection 
illuminating system for illuminating the sample for the inspection of the 
surface provided with a circuit pattern, capable of illumination for a 
large depth of focus, instead of low light concentration, to utilize the 
time to spare permitted by the low-sensitivity detection. 
When a focusing optical system having a large numerical aperture is used, 
it is usual to change resolution by changing the magnification of the 
objective. Since the front surface of the substrate provided with a 
circuit pattern, the back surface of the substrate, and the pellicle 
extend respectively in different planes, the focus (detection point) must 
be moved in a range of several millimeters and hence the Z-stage, i.e., a 
stage that moves along the width of the photomask, such as a reticle, must 
be moved accurately for the high-accuracy high-resolution inspection of 
the circuit pattern. 
The present invention employs a detecting unit for inspecting the back 
surface of the reticle and the pellicle at a high detecting speed in a low 
resolution in addition to a detecting unit for inspecting the front 
surface of the reticle on which a circuit pattern is formed. 
(8) Shading Correcting Method: Detection Wavelength Determining Method: 
Detection Sensitivity Determining Method

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A reticle as shown in FIG. 36 has been recently developed to improve 
resolution in transferring a circuit pattern of a metal thin film, such as 
a chromium thin film (hereinafter referred to as "chromium pattern"). This 
recently developed reticle will be referred to as "phase shift reticle" 
hereinafter. The phase shift reticle is provided with a pattern of a 
transparent or translucent thin film (hereinafter referred to as "phase 
shift pattern"), which is called a phase shift film or a phase shifter, 
having a thickness equal to odd times the half of the wavelength of light 
emitted by an exposure light source. The film forming the phase shift 
pattern is transparent or translucent and has a thickness several times 
the thickness on the order of 0.1 .mu.m of the chromium pattern. 
When inspecting a reticle by the conventional reticle inspecting apparatus, 
the front surface of the reticle on which the chromium pattern is formed 
is illuminated and scattered light is gathered by a detection optical 
system disposed on the side of the front surface (FIG. 16, front 
illumination mode). When the phase shift reticle is inspected for foreign 
particles in this front illumination mode, there arises a problem that 
scattered light scattered by edges of the phase shifter pattern, which is 
several times to several tens times greater than diffracted light 
diffracted by edges of the chromium pattern, reduces the foreign particle 
detection sensitivity greatly. 
This invention utilizes a fact that the edges of the phase shifter pattern 
extend on the chromium pattern of an opaque film to solve the problem. 
When the illuminating light is projected from the side of the back surface 
of the reticle and the scattered light is gathered by the detection 
optical system disposed on the side of the front surface of the reticle 
(FIG. 17, back illumination mode), the illuminating light traveling toward 
the edges of the phase shift pattern is intercepted by the chromium 
pattern of an opaque film of the phase shift reticle and, consequently, 
the foreign particle detection sensitivity is not reduced because the 
illuminating light is not scattered by the phase shift pattern. 
The back illumination mode is able to detect only foreign particles in the 
light-transmissive portions, i.e., portions in which any elements of the 
chromium pattern are not formed. Practically, foreign particles on the 
chromium pattern needs to be detected. Accordingly, it is desirable to 
illuminate the reticle in both the front illumination mode and the back 
illumination mode in combination. The front illumination mode and the back 
illumination mode will be described hereinafter in terms of foreign 
particles and the intensity of the scattered light scattered by the front 
surface of the reticle. 
According to the light scattering theory, scattered light scattered by 
particles is in similar correspondence in respect of the relation between 
wavelength and particle size. FIG. 18 shows the relation between the 
distribution of scattered light scattered by a particle and d/.lambda. (d: 
size of the particle, .lambda.: wavelength of the light emitted by a light 
source). The light component scattered in the direction of travel of the 
illuminating light is called a forward scattered light component, and the 
light component scattered in the direction opposite the direction of 
travel of the illuminating light is called a backward scattered light 
component. 
When the illuminating light falls on a particle having a certain size, the 
shorter the wavelength of the illuminating light, the greater is the 
forward scattered light component, and the longer the wavelength of the 
illuminating light, the higher is the uniformity of distribution of the 
scattered light components and the greater is the ratio of the backward 
scattered light component to all the scattered light components. 
FIG. 19(A) shows the positional relation between the direction of travel of 
the illuminating light and the detection optical system for the front 
illumination mode, and FIG. 19(B) shows the positional relation between 
the direction of travel of the illuminating light and the detection 
optical system for the back illumination mode. The front illumination mode 
detects the backward scattered light component, and the back illumination 
mode detects the forward scattered light component. As shown in FIG. 18, 
the forward scattered light component is always greater than the backward 
scattered light component. Therefore, it is effective to detect the 
forward scattered light component to obtain a high foreign particle 
detection signal. Thus, it is advantageous to detect the forward scattered 
light component in the back illumination mode to detect foreign particles 
in the light-transmissive portions of the reticle whether the reticle is 
provided with the phase shifter pattern or not. 
A reticle inspecting apparatus for inspecting a reticle, such as a 
photomask, fabricated by forming a circuit pattern of an opaque film on a 
transparent (or translucent) substrate for foreign particles adhering to 
the substrate is able to provide high foreign particle detection signals 
by detecting foreign particles in the opaque portions in the front 
illumination mode and detecting foreign particles in the 
light-transmissive portions in the back illumination mode. 
In either illumination mode, a maximum foreign particle detection signal 
can be obtained by using illuminating light having an optimum wavelength. 
Experiments were conducted to determine illuminating light having an 
optimum wavelength to obtain a maximum foreign particle detection signal 
through the examination of the dependence of detecting ability on the 
wavelength of illuminating light. 
In the front illumination mode, the backward scattered light component and 
hence the foreign particle detection signal increase with the increase of 
the wavelength of the illuminating light. 
FIG. 20 shows the variation of chromium pattern detection signal provided 
when the chromium pattern is detected and the variation of the particle 
detection signal provided when a 0.5 .mu.m particle on the chromium 
pattern (opaque portion) is detected in the front illumination mode with 
the wavelength of the illuminating light. Laser beams of 830 nm, 780 nm, 
633 nm, 532 nm, 515 nm and 488 nm, respectively, in wavelength were used 
as illuminating light beams. In the wavelength range of 488 nm to 830 nm, 
the longer the wavelength, the higher is the particle detection signal, 
and the particle detection signal reaches a peak when the wavelength is 
780 nm. The chromium pattern detection signal varies with the wavelength 
in a comparatively narrow range. 
FIG. 21 shows the variation of a shifter pattern detection signal provided 
when the shifter pattern is detected and the variation of the particle 
detection signal provided when a 1.0 .mu.m particle on the chromium 
pattern (opaque portion) is detected in the front illumination mode with 
the wavelength of the illuminating light. Laser beams of 830 nm, 780 nm, 
633 nm, 532 nm, 515 nm and 488 nm, respectively, in wavelength were used 
as illuminating light beams. In the wavelength range of 488 nm to 830 nm, 
the longer the wave-length, the higher are both the particle detection 
signal and the shifter pattern detection signal. 
In the back illumination mode, the shorter the wavelength of the 
illuminating light, the greater is the forward scattered light component 
and higher is the particle detection signal. 
FIG. 22 shows the variation of the particle detection signal provided when 
a 0.5 .mu.m particle on the glass plate (light-transmissive portion) is 
detected and the variation of chromium pattern detection signal provided 
when the chromium pattern is detected in the back illumination mode with 
the wavelength of the illuminating light. In the back illumination mode, 
the shifter pattern does not scatter the illuminating light at all. Laser 
beams of 780 nm, 633 nm, 532 nm, 515 nm and 488 nm, respectively, in 
wavelength were used as illuminating light beams. The shorter the 
wavelength, the higher is the particle detection signal. Although the 
shorter the wavelength, the higher the chromium pattern detection signal, 
the chromium pattern detection signal varies with the wavelength less 
sharply than the particle detection signal. 
When inspecting a sample provided with a circuit pattern for foreign 
particles, the relation between the foreign particle detection signal 
provided when scattered light scattered by a foreign particle is detected 
and the pattern detection signal provided when scattered light scattered 
by the circuit pattern is detected must be taken into consideration. This 
relation is represented by discrimination ratio defined by: 
EQU (Discrimination ratio)=(Output of the detector provided when scattered 
light scattered by a foreign particle is detected)/(Output of the detector 
provided when scattered light scattered by the pattern is detected) 
If the discrimination ratio is greater than "1", the foreign particle can 
be detected through the comparison (binarization) of the scattered light 
detection signals by an apparatus having a simple configuration. In a 
practical apparatus, the detection signal is affected by electrical noise, 
optical noise, vibrations of the mechanical parts, the variation of 
sensitivity of the detection system and such. Therefore, there must be a 
significant difference between the level of scattered light scattered by 
the foreign particle and that of scattered light scattered by the chromium 
pattern; that is, the greater the discrimination ratio, the higher is the 
foreign particle detecting ability. 
The foregoing experimental results were examined to determine the 
wavelengths of illuminating light beams to enhance the detecting abilities 
of the front illumination mode and the back illumination mode to a 
maximum. 
FIGS. 23 and 24 shows the variation of the discrimination ratio with the 
wavelength of the illuminating light beam in the inspection in the front 
illumination mode. 
(1) FIG. 23: A 0.5 .mu.m standard particle on the chromium pattern vs 
chromium pattern (Maximum value) 
(2) FIG. 24: A 1.0 .mu.m standard particle on the chromium pattern vs 
shifter pattern (Maximum value) 
It is known from FIG. 23 that the 0.5 .mu.m standard particle on a reticle 
not provided with any phase shift film can be most stably detected when an 
illuminating light beam having a wavelength around 780 nm is used. 
It is known from FIG. 24 that the 1.0 .mu.m standard particle on the 
chromium pattern of a phase shift reticle can be detected by using an 
illuminating light beam having a wavelength in the range of 600 nm to 800 
nm. 
From these facts known from FIGS. 23 and 24, it is considered that an 
illuminating light beam having a wavelength around 780 nm is an optimum 
illuminating light beam for the front illumination mode. 
A light source capable of emitting such an optimum illuminating light beam 
having a wavelength around 780 nm is a semiconductor laser. It is obvious 
from FIG. 23 that the discrimination ratio achieved by using this optimum 
illuminating light beam is higher than that achieved by using a laser beam 
having a wavelength of 632.8 nm emitted by a red He--Ne laser, which has 
been widely used, and the optimum illuminating light beam secures stable 
foreign particle detection. 
FIG. 25 shows the variation of the discrimination ratio with the wavelength 
of the illuminating light beam in the inspection in the back illumination 
mode. 
(1) FIG. 25: A 0.5 .mu.m standard particle on the glass substrate vs the 
chromium pattern (Maximum value) 
It is known from FIG. 25, the discrimination ratio reaches a maximum when 
an illuminating light beam having a wavelength of around 488 nm is used in 
the inspection in the back illumination mode. 
A light source that emits light having a wavelength of about 488 nm is the 
Ar ion laser. The Ar ion laser having a large output capacity can be 
easily fabricated; an air-cooled Ar ion laser can provide an output as 
large as several tens of milliwatts and a water-cooled Ar ion laser can 
provide an output as large as several watts. Therefore, the detection 
signal when an Ar ion laser beam is used is higher than that when a red 
He--Ne laser beam is used. 
Thus, from the foregoing, the present invention uses oblique illumination 
by an illuminating light beam having a wavelength of about 780 nm for the 
front illumination mode and oblique illumination by an illuminating light 
beam having a wavelength of about 488 nm in combination for the 
discriminative detection of foreign particles and a circuit pattern on a 
sample provided with a phase shift film. 
The foregoing optimum wavelengths are selected on an assumption that the 
size of a minimum foreign particle among those to be detected is 0.5 
.mu.m. Since the greater the size of foreign particles, the higher is the 
detection signal, i.e., the amount of scattered light, a wavelength that 
makes the detection signal provided when a foreign particle having the 
minimum size a maximum is detected is the optimum wavelength. Since 
scattering is in similar correspondence in respect of the relation 
d/.lambda. (d is the size of the particle and .lambda. is the wavelength 
of the illuminating light beam). Accordingly, from the foregoing 
experimental results, an optimum wavelength is on the order of 1.6 d for 
the front illumination mode and on the order of 1.0 d for the back 
illumination mode, where d is the size of the smallest foreign particle 
among those to be detected. 
Although the backward scattered light component increases if an 
illuminating light beam having a wavelength greater than the optimum 
wavelength is used for the front illumination mode, the total amount of 
scattered light decreases in inverse proportion to the fourth power of the 
wavelength of the illuminating light beam (Reyleigh scattering), entailing 
the reduction of the particle detection signal. If an illuminating light 
beam having a wavelength smaller than the optimum wavelength for the back 
illumination mode is used for oblique illumination, the forward scattered 
light component increases excessively and the amount of light that falls 
on the detection optical system decreases, reducing the particle detection 
signal. When the size of the smallest foreign particle among those to be 
detected is 0.5 .mu.m, the wavelength for the front illumination mode must 
be in the range of 600 nm to 800 nm and the wavelength for the back 
illumination mode must be in the range of 450 nm to 550 nm. 
Referring to FIG. 1, an inspection stage unit 1 comprises a Z-stage 10 
provided with a pellicle 7 and capable of being moved in the Z-direction, 
a fastening device 18 for fastening a reticle 6 on the Z-stage; an X-stage 
11 for moving the Z-stage 10 supporting the reticle 6 in the X-direction; 
a Y-stage 12 for moving the Z-stage 10 supporting the reticle 6 in the 
Y-direction, a stage driving system 13 for driving the Z-stage 10, the 
X-stage 11 and the Y-stage 12 for movement, and a control system 14 for 
detecting the position of the reticle 6 with respect to the Z-direction to 
position the reticle 6 for focusing. The stages 10, 11 and 12 are 
controlled in a necessary accuracy for focusing during the inspection of 
the reticle 6. 
The X-stage 11 and the Y-stage 12 are controlled for movement for scanning 
along scanning lines as shown in FIG. 2 at an optional moving speed. For 
example, the X-stage is driven for a periodic movement in a half-cycle 
time of about 0.2 sec for uniformly accelerated motion, 4.0 sec for 
uniform motion, 0.2 sec for uniformly decelerated motion and about 0.2 for 
stopping, at a maximum velocity of about 25 mm/sec in an amplitude of 105 
mm. The Y-stage 12 is driven for intermittent movement in the Y-direction 
in synchronism with the uniformly accelerated motion and the uniformly 
decelerated motion of the X-stage 11 at a step of 0.5 mm. If the Y-stage 
12 is moved 200 times at a step of 0.5 mm, the reticle 6 can be moved 100 
mm in about 960 sec and an area of 100 mm square can be scanned in about 
960 sec. 
The stage driving system 13 may be provided with an air micrometer, a laser 
interferometer or a device employing a stripe pattern to position the 
reticle 6 for focusing. In FIGS. 1 and 2, the X-direction, the Y-direction 
and the Z-direction are indicated by the arrows X, Y and Z, respectively. 
The reticle inspecting apparatus has a first front illuminating unit 2, a 
second front illuminating unit 20, a first back illuminating unit 3 and a 
second back illuminating unit 30, which are individual systems and the 
same in configuration. The front illuminating units 2 and 20 are provided 
respectively with laser light sources 21 and 201 which emit light beams of 
780 nm in wavelength. The back illuminating units 3 and 30 are provided 
respectively with laser light sources 31 and 301 which emit light beams of 
488 nm in wavelength. Laser beams emitted by the laser light sources 21, 
201, 31 and 301 are condensed respectively by condenser lenses 22, 202, 32 
and 302 to illuminate a circuit pattern formed on the front surface of the 
reticle 6. The incidence angle i of each of the light beams emitted by the 
laser light sources 2, 20, 3 and 30 on the circuit pattern must be greater 
than about 30.degree. to avoid the collision of the light beam on the 
objective lens 41 of a detection optical system 4 and must be smaller than 
about 80.degree. to avoid the collision of the same on the pellicle 7 
mounted on the reticle 6. Therefore, about 30.degree.&lt;i&lt;about 80.degree.. 
The illuminating units are provided respectively with shutters 23, 203, 33 
and 303 which open to pass the light beams emitted by the corresponding 
light sources and close to cut off the same, respectively, and, if 
necessary, can be operated individually. 
Since the first front illuminating unit 2, the second front illuminating 
unit 20, the first back illuminating unit 3 and the second back 
illuminating unit 30 are the same in configuration, only the first front 
illuminating unit 2 will be described with reference to FIG. 3, in which 
parts like or corresponding to those shown in FIG. 1 are denoted by the 
same reference characters. The first front illuminating unit 2 is provided 
with the condenser lens 22 consisting of a convex lens 223, a cylindrical 
lens 224, a collimator lens 225 and a condenser lens 226. 
The laser light sources 21 and 201 of the front illuminating units 2 and 20 
are disposed so that light beams emitted by the laser light sources 21 and 
201 are linearly polarized light beams (s-polarized light beams) having 
the electric vector which remains pointing to the X'-direction. The 
s-polarized light beams are used because the reflectivity of the 
s-polarized light beams incident on a glass substrate at an incidence 
angle i of about 60.degree. is about five times greater than that of the 
p-polarized light beams (linearly polarized light having the electric 
vector which remains pointing to the Y'-direction) and hence the 
s-polarized light beams are more suitable for detecting small particles 
than the p-polarized light beams. 
The laser light sources 31 and 301 of the back illuminating units 3 and 30 
are disposed so that light beams emitted by the laser light sources 31 and 
301 are s-polarized light beams because experiments showed that the 
discrimination ratio when the s-polarized light beam is used is greater 
than the discrimination ratio when the p-polarized light beam is used. 
However, in some cases, the p-polarized light beam is preferred to the 
s-polarized light beam, taking into consideration the transmittance of the 
substrate. 
The present invention uses spatial filters disposed on the Fourier 
transform plane of the detection optical system 4 to discriminate between 
foreign particles and the circuit pattern. The use of a collimated light 
beam reduces the spread of diffracted light diffracted by the circuit 
pattern to increase the discrimination ratio. However, the use of gathered 
light of high intensity will raise the output level of the detector and 
improve the SN ratio. 
If the NA of the converging system is about 0.1 and the diameter of the 
laser beam is reduced to about 10 .mu.m to increase the intensity of the 
laser beam emitted by each of the illuminating units 2, 20, 3 and 30, the 
depth of focus is as small as about 30 .mu.m, which is smaller than the 
size (500 .mu.m) of the entire area S of an inspection field (FIG. 3) and 
the entire area S of the inspection field 15 cannot be brought into focus. 
In this reticle inspecting apparatus, the cylindrical lens 214 is turned 
about the X'-axis as shown in FIG. 3 to bring the entire area S of the 
inspection field 15 into focus when the incidence angle i is, for example, 
60.degree.. Accordingly, even if the detectors 51 and 551 of a signal 
processing system 5 are one-dimensional solid-state imaging devices and 
the inspection area of the inspection field 15 has a linear shape, the 
linear inspection area can be uniformly illuminated in a high illuminance. 
When the cylindrical lens 224 is turned about both the X'-axis and the 
Y'-axis (FIG. 3), the entire area S of the inspection field 15 can be 
uniformly and linearly illuminated in a high illuminance even if the light 
beam is projected from an optional direction so as to fall on the reticle 
at an incidence angle i of 60.degree.. 
The shutters 23 and 203 intercept the light beams emitted by the laser 
light sources 21 and 201, respectively, when necessary. The following 
cases need the control of the light beams by the shutters. FIGS. 38(A) to 
38(C) show the relation between the reticle 6, the pellicle 7, the oblique 
illuminating light beam 3802 projected by the illuminating unit 2, an 
oblique illuminating light beam 3820 projected by the illuminating unit 
20, an oblique illuminating light beam 3803 projected by the illuminating 
unit 3, an oblique illuminating light beam 3830 projected by the 
illuminating unit 30 and an inspection field 15, i.e., an illuminated 
position. When the stage is moved in the positive direction along the 
Y-axis from the position shown in FIG. 38(B), the illuminating light beam 
3820 projected by the illuminating unit 20 is intercepted by the frame 
3807 of the pellicle 7 mounted on the reticle 6 upon the arrival of the 
stage at a position shown in FIG. 38(A). When the stage is moved in the 
negative direction along the Y-axis from the position shown in FIG. 38(B), 
the illuminating light 3802 projected by the projected by the illuminating 
unit 2 is intercepted by the frame 3807 of the reticle 7 upon the arrival 
of the stage at a position shown in FIG. 38(C). When the stage is at the 
position shown in FIG. 38(A) or 38(C), part of the illuminating light is 
obstructed, the illuminance in the inspection field 15 is reduced and the 
degree of reduction varies every moment, which makes stable illumination 
impossible. Moreover, part of the obstructed illuminating light beams 
become stray light which affects adversely to defect detection. Therefore, 
the illuminating light beam 3820 (3802) must be cut off by the shutter 203 
(23) before the same is obstructed. Thus, the illuminated area which can 
be illuminated by the illuminating system 2 (20) is dependent on the 
relation between the incidence angle of the illuminating light beam 3802 
(3820) and the frame 3807. Referring to FIG. 37 showing illuminated areas, 
an area 3704 is illuminated in an illuminating mode shown in FIG. 38(B), 
an area 3724 is illuminated in an illuminating mode shown in FIG. 38(C), 
an area 3704 is illuminated in an illuminating mode shown in FIG. 38(A), 
when the frame 3807 is 102 mm long, 102 mm wide and 6.3 mm high, and the 
angle between the optical axis of each illuminating unit and the front 
surface of the reticle 6 provided with the circuit pattern is 30.degree.. 
As shown in FIG. 37, the entire area 3701 of the surface of a chip for a 
64M DRAM is included in the area 3704. 
The same illuminating modes as the foregoing illuminating modes concerning 
the front illuminating units 2 and 20 occur to the back illuminating units 
3 and 30 when the same frame 3807 holding a pellicle is attached to the 
back surface of the reticle 6. 
Although the back illuminating units 3 and 30 of the reticle inspecting 
apparatus of FIG. 1 are provided respectively with the small laser light 
sources 31 and 301 having a comparatively small output capacity, the two 
small laser light sources 31 and 301 may be substituted by a single large 
laser light source having a comparatively large output capacity and the 
laser beam emitted by the large laser light source may be divided into two 
laser beams as shown in FIG. 54. A back illuminating unit shown in FIG. 54 
corresponds to the back illuminating units 3 and 301 shown in FIG. 1. 
Since the laser beam emitted by a single laser light source 5401 is 
divided into two laser beams, the back illuminating unit shown in FIG. 54 
has a comparatively long optical path length and, consequently, the laser 
beam emitted by the laser light source 5401 is liable to be disturbed and 
to expand while traveling along the long optical path. Therefore, the 
laser beam is expanded by a beam expander 5402 to increase its diameter, 
and then the expanded laser beam is divided into two laser beams by a beam 
divider 5403 so that the two laser beams travel along two optical paths, 
respectively. One of the optical paths corresponds to the back 
illuminating unit 3 of FIG. 1, and a shutter 33, reflecting mirrors 5406, 
5407 and 5408, and a condenser lens 32 are arranged on the optical path. 
The laser light beam is guided by the reflecting mirrors 5406, 5407 and 
5408 to the condenser lens 32, and then the condenser lens 32 concentrates 
the laser beam on a reticle 6. This back illuminating unit is only an 
example and may be substituted by any suitable back illuminating unit of 
another configuration. When the laser beam is a linearly polarized laser 
beam, the reflecting mirrors arranged on the optical path must be such as 
will not affect adversely to the plane of polarization of the laser beam. 
The beam divider 5403 may be of a transmittance division type, a 
polarization division type or, if the laser light source is such as is 
capable of emitting light having a plurality of wavelengths, such as an 
argon laser, a wavelength division type. Desirably, the laser beam emitted 
by the laser light source 5401 is divided into two laser beams having 
equal intensities. When it is difficult to divide the laser beam into two 
equal laser beams, the intensities of the two laser beams as divided may 
be adjusted to equal intensities by variable ND filters 5409 and 5410 
provided respectively on the branch optical paths as shown in FIG. 55. 
When the laser beam is divided by polarization, half-wave plates 5414 and 
5412 are provided respectively on the two branch optical paths as shown in 
FIG. 55 in order that the respective planes of polarization of the two 
laser beams on the surface of the reticle 6 are the same. The purity of 
polarization of the two laser beams may be improved by polarizers 5415 and 
5413. 
When the laser beam emitted by the single laser light source is divided 
into two laser beams, the two laser beams are transmitted along two branch 
optical paths, and an inspection field is illuminated with the two laser 
beams, the two laser beams interfere with each other on the surface of the 
reticle 6 to form interference fringes and, consequently, the inspection 
field is illuminated very irregularly. In such a case, the two branch 
optical paths are formed so that the optical path difference between the 
two branch optical paths is not shorter than the coherence length, for 
example a length in the range of several millimeters to several meters, 
for the laser beam emitted by the laser of the laser light source. When 
the beam divider of a wavelength division type is employed, the two laser 
beams do not interfere with each other and hence the influence of 
interference between the two laser beams traveling along the two branch 
optical paths need not be taken into consideration. If the oscillation 
wavelengths of 488 nm and 515 nm among those of an argon laser are used, 
detection sensitivities with 488 nm and 515 nm are not greatly different 
from each other because the difference between 488 nm and 515 nm is small, 
and irregular detection sensitivity caused by the effect of interference, 
which is difficult to analyze, can be obviated by a minute wavelength 
difference caused by the shape of a defect, such as a foreign particle. 
In the back illuminating mode, the optical path length of an optical path 
along which the illuminating light beam travels is dependent on the 
thickness of the glass plate, i.e., the substrate of the reticle. Even if 
the optical path is formed so that the illuminating light beam falls on 
the inspection field 15 on a reticle 5601 of substantially zero in 
thickness as shown in FIG. 56(1), the illuminating light beam travels 
through a reticle 5602 having a comparatively small thickness along a path 
5612 deviating from a path 5622 reaching the inspection field 15 and 
illuminates an illuminated position dislocated by an error E2 from the 
inspection field 15 as shown in FIG. 56(2). Similarly, the illuminating 
light beam travels through a reticle 5603 having a middle thickness along 
a path 5613 deviating from a path 5623 reaching the inspection field 15 
and illuminates an illuminated position dislocated by an error E3 as shown 
in FIG. 56(3), and the illuminating light beam travels through a reticle 
5604 having a comparatively large thickness along a path 5614 deviating 
from a path 5624 reaching the inspection field 15 and illuminates an 
illuminated position dislocated by an error E4 as shown in FIG. 56(4). 
Since photomasks, such as reticles are formed on substrates having 
different thicknesses, such as 2.3 mm, 4.6 mm and 6.3 mm, effective 
measures must be taken to prevent the dislocation of an illuminated 
position from a desired inspection field. 
Although the dislocation of the illuminated position from the inspection 
field 15 attributable to the effect of the thickness of the substrate is 
not any problem if an illuminating light beam capable of illuminating a 
wide area on the reticle including the errors E2 to E4 is used, the use of 
such an illuminating light beam reduces the illuminance of the inspection 
field and reduces the S/N ratio. FIGS. 57(1) to 57(4) shows means for 
illuminating the inspection field 15 regardless of the thickness of the 
reticle. An illuminating light beam is transmitted so as to travel through 
the reticle 5602 along an optical path 5712 as shown in FIG. 57(2), 
through the reticle 5603 along an optical path 5713 as shown in FIG. 57(3) 
and through the reticle 5604 along an optical path 5714 as shown in FIG. 
57(4) so that the illuminating light beam falls on a inspection field 
corresponding to the inspection field 15 on a reticle of virtually zero in 
thickness on which the illuminating light beam will fall when the 
illuminating light beam travels along a straight optical path 5611. FIG. 
58 shows a back illuminating unit provided, in addition to the components 
of the back illuminating unit of FIG. 55, with optical path shifting 
devices 5801 and 5811, and driving mechanisms 5802 and 5812 for driving 
the optical path shifting devices 5801 and 5811 to shift the optical paths 
according to the thickness of the reticle. FIG. 59 shows a back 
illuminating unit provided, in addition to the components of the back 
illuminating unit of FIG. 55, with angular position changing devices 5901 
and 5911 for changing the angular positions of the reflecting mirrors 5408 
and 5405, and driving mechanisms 5902 and 5912 for driving the angular 
position changing devices 5901 and 5911 to change the illuminating angle 
according to the thickness of the reticle. 
The influence of the difference in the thickness of the reticle on the 
optical path is dependent on the difference in refractive index between 
the substrate of the reticle and the optical path (the influence 
disappears if the difference in refractive index is zero). The difference 
in refractive index brings about optical path difference in the focusing 
system of the illuminating unit. That is, when focusing an illuminating 
light beam the difference in thickness of the reticle affects focusing 
and, if the depth of focus of the focusing system is not sufficiently 
large, focus must be adjusted to adjust the focusing position, which 
requires a complicated apparatus. However, if the change in the optical 
path length due to the difference in thickness of the reticle is corrected 
by some means, the optical path shifting device and the angular position 
changing devices as shown in FIGS. 58 and 59 are unnecessary. FIGS. 60(1) 
to 60(4) illustrates a principle of correcting optical path length. When 
illuminating reticles respectively of 0, t2, t3 and t4 in thickness, an 
optical path length correcting plate of t4 in thickness is disposed under 
the reticle of 0 in thickness, an optical path length correcting plate of 
(t4-t2) is disposed under the reticle of t2 in thickness, an optical path 
length correcting plate of (t4-t3) in thickness is disposed under the 
reticle of t3 in thickness, and no optical path length correcting plate is 
disposed under the reticle of t4 in thickness, so that all the optical 
path lengths respectively for the reticles of 0, t2, t3 and t4 in 
thickness are equal to each other, and the illuminated positions and focal 
positions on all the reticles correspond exactly to each other. FIG. 61 
shows a back illuminating unit provided, in addition to the components of 
the back illuminating unit of FIG. 55, with optical path length correcting 
units 6101 and 6111 each provided with a plurality of optical path length 
correcting plates of different thicknesses, and driving mechanisms 6102 
and 6112 for driving the optical path length correcting units 6101 and 
6111. The optical path length correcting units may be such as are provided 
with liquid means capable of being deformed, or are capable of 
continuously changing optical path length by an electrooptic means. 
The reticle inspecting apparatus of FIG. 1 is provided with a detection 
optical system 4 for measuring the transmittance of the pellicle. The 
transmittance of the pellicle is affected slightly by the thickness of the 
pellicle and the antireflection film coating the pellicle. The influence 
of the thickness and the antireflection film on the transmittance is 
insignificant with light that penetrates the pellicle perpendicularly, 
such as the scattered light from the reticle. However, in some cases, the 
influence of the thickness and the antireflection film on the 
transmittance is significant with light that penetrates the pellicle 
obliquely, such as the illuminating light beam for illuminating the 
reticle. This problem can be solved by measuring the transmittance of the 
pellicle of each reticle and correcting the results of detection by using 
the measured transmittance of the pellicles. However, the transmittance of 
the pellicle cannot directly be measured because the pellicle is mounted 
on the reticle. Therefore, the detection optical system 4 measures the 
intensity of the illuminating light beam projected by the illuminating 
unit and reflected from the pellicle and that of the illuminating light 
beam as emitted by the light source, determines the reflectance of the 
pellicle on the basis of the measured data, and calculates transmittance 
by using: (Transmittance)=1-(Reflectance). Then, the results of detection 
are corrected by using the calculated transmittance of the pellicle. 
Referring again to FIG. 1, the detection optical system 4 comprises the 
objective lens 41 disposed opposite to the front surface of the reticle 6, 
a field lens 43 disposed near the focal point of the objective lens 41, 
and a wavelength separating mirror 42. The light incident on the detection 
optical system 4 is separated into a scattered light component and a 
diffracted light of the front illuminating units 2 and 20, and those of 
the back illuminating units 3 and 30. The separated light components 
travel through spatial filters 44 and 444 disposed on Fourier transform 
planes with respect to the inspection field 15 on the reticle 6 and each 
having a band-like screening portion and light-transmissive portions on 
the opposite sides of the band-like screening portion, and focusing lenses 
45 and 445 and form images of the inspection field 15 on the reticle 6 on 
the detectors 51 and 551 of the signal processing system 5, respectively. 
The field lens 43 forms an image of a focus position 46 above the 
objective lens 41 on the spatial filters 44 and 444. 
FIG. 17 shows a reticle inspecting apparatus of the back illuminating unit. 
The respective positions of an illuminating unit 3 and a detection optical 
system 4 included in the reticle inspecting apparatus may be interchanged 
with respect to the reticle 6. FIG. 34 shows another reticle inspecting 
apparatus of the back illuminating unit, in which the respective positions 
of an illuminating unit 31 and a detection optical unit 40 with respect to 
the reticle 6 are reverse to those of the illuminating unit 3 and the 
detection optical system 4 of the reticle inspecting apparatus of FIG. 17. 
The reticle inspecting apparatus of FIG. 17 detects scattered light 
scattered by a foreign particle on the transparent substrate of the 
reticle 6, and the reticle inspecting apparatus of FIG. 34 detects 
scattered by a foreign particle and transmitted through the transparent 
substrate of the reticle 6. When the scattered light transmitted through 
the transparent substrate of the reticle 6 as in the reticle inspecting 
apparatus of FIG. 34, the resolution is deteriorated by aberration caused 
by the substrate of the reticle 6, which makes the stable detection of the 
foreign particle difficult. Therefore, the image forming optical system of 
the reticle inspecting apparatus of FIG. 34 needs to be provided with a 
lens capable of compensating the aberration caused by the substrate of the 
reticle 6. 
A reticle inspecting apparatus shown in FIG. 35 analogous with the reticle 
inspecting apparatus shown in FIG. 34 in configuration. The reticle 
inspecting apparatus of FIG. 35 is suitable for inspecting the entire 
surface of the reticle 6. The reticle inspecting apparatus shown in FIG. 
35 comprises a first front illuminating unit 21, a second front 
illuminating unit 31, which are disposed on the side of the front surface 
of the reticle 6, a front detection optical system 4 disposed on the side 
of the front surface of the reticle 6, and a back detection optical system 
40 disposed on the side of the back surface of the reticle 6. The front 
detection optical system 4 detects scattered light scattered by opaque 
portions of the reticle 6, i.e., reflected light, and the back detection 
optical system 40 detects scattered by light-transmissive portions of the 
reticle 6, i.e., transmitted light. The front detection optical system 4 
and the back detection optical system 40 must be provided respectively 
with appropriate wavelength filters to detect only reflected light and 
only transmitted light, respectively. 
Foreign particles on the chromium pattern, i.e., the opaque film, of the 
reticle do not cause defects when an image of the reticle is printed by a 
photographic process. Foreign particles on exposed portions of the glass 
substrate cause defects in a photographically printed image of the 
reticle. Accordingly, foreign particles which may migrate from the 
chromium pattern to positions outside the chromium pattern, migratory 
foreign particles, must be detected. 
Defects, such as foreign particles, on the chromium pattern must be 
detected in addition to the migratory foreign particles in the following 
cases. 
In some cases, foreign particles on the chromium pattern cause problems 
when fabricating a phase shift reticle. Generally, when fabricating a 
phase shift reticle, a circuit pattern of a chromium film, i.e., a 
chromium pattern, is formed on the front surface of a substrate, a phase 
shift film is formed over the entire area of the front surface of the 
substrate by coating or sputtering, and then the phase shift film is 
etched to form a phase shift pattern. If there are foreign particles on 
the chromium pattern in forming the phase shift film, voids and breaks are 
formed in the phase shift film and, in some cases, the voids and breaks 
cause faulty printing. Therefore, the entire area of the substrate 
including the surface of the chromium pattern must be inspected before 
forming the phase shift film. The reticle inspecting method in accordance 
with the present invention is capable of detecting defects, such as voids 
and breaks, as well as foreign particles. 
The entire surface of a transparent or translucent substrate not provided 
with a circuit pattern can be inspected by the reticle inspecting 
apparatus shown in FIG. 17 or 34. Since there is no circuit pattern and 
hence no diffracted light diffracted by a circuit pattern, the reticle 
inspecting apparatus need not be provided with the spatial filter 44. When 
the front scattered light is detected by this reticle inspecting 
apparatus, the magnitude of a detection signal provided upon the detection 
of a foreign particle is higher than that of a detection signal provided 
by a reticle inspecting apparatus of a reflection illumination system. If 
the spatial filter 44 is omitted, the stage may be moved for XY scanning 
or rotational scanning. 
Accordingly, it is desirable to provide the reticle inspecting apparatus 
with a mechanism capable of setting the spatial filter at the working 
position when necessary. The reticle inspecting apparatus may be provided 
with a spatial filter unit having a plurality of spatial filters capable 
of being selectively set at the working position one at a time. FIG. 63 
shows a filter unit provided with a slot holding a linear spatial filter, 
a slot holding a combination of a linear spatial filter and a polarizing 
plate and two spare slots. A reticle inspecting apparatus shown in FIG. 64 
is provided with filter units 6401 and 6411, which are similar to the 
filter unit of FIG. 63, and driving mechanisms 6402 and 6412 respectively 
for moving the filter units 6401 and 6411 to locate a selected spatial 
filter at a working position. In FIG. 64, a signal processing unit is 
omitted. 
To inspect an object, such as a reticle, in which the circuit pattern 
forming condition and required sensitivity for detection change in each 
process, a reticle inspecting apparatus capable of changing sensitivity 
for detection for each process is used. 
A signal processing system 5 as shown in FIG. 1, for example, comprises 
detectors 51 and 551, shading compensating circuits 113 and 123 for 
correcting the output signals of the detectors 51 and 551, four-pixel 
addition circuits 114 and 124, binarizing circuits 52, 53, 552 and 553, OR 
circuits 56 and 556, an AND circuit 57, block processing circuits 58 and 
558, a microcomputer 54 and a display 55. 
Each of the detectors 51 and 551 is, for example, a one-dimensional 
solid-state imaging device of a charge transfer type. When a defect, such 
as a foreign particle, is found in the inspection field 15 while the 
circuit pattern of the reticle 6 is scanned by moving the X-stage 10, the 
level of the light signal representing the circuit pattern, i.e., the 
intensity of the incoming light, increases and, consequently, the outputs 
of the detectors 51 and 551 increases. The one-dimensional solid-state 
imaging device is advantageous because the inspection field 15 can be 
expanded without reducing the resolution. The detectors 51 and 551 may be 
two-dimensional solid-state imaging devices or solid-state image sensors. 
A binarizing threshold is set for the binarizing circuits 52 and 552. When 
the binarizing circuits 52 and 552 receive outputs of the detectors 51 and 
551 exceeding a level corresponding to the intensity of reflected light 
corresponding to a size of a foreign particle to be detected, the 
binarizing circuits 52 and 552 provide logical "1". The outputs of the 
detectors 51 and 551 are provided together with the logic level, i.e., the 
result of binarization, to use the outputs for estimating the size and the 
like or to facilitate setting a threshold for binarization. 
Shading compensating circuits 113 and 123 and 4-pixel addition circuits 114 
and 124 will be described later. A blocking circuit 112 receives the 
output signals of the binarizing circuits 52 and 552 and prevents the 
double count of the two signals, which will be described later. 
Upon the reception of a logical "1" from the blocking circuit 112, the 
microcomputer 54 decides that a defect is found, stores defect data 
including information about the respective positions of the X-stage 10 and 
the Y-stage 11, information about the position of the defect determined by 
calculation on the basis of the pixels, i.e., the solid-state image 
sensors, of the detectors 51 and 551 corresponding to the defect, and the 
values of the outputs of the detectors 51 and 551, and displays the defect 
data on the display 55. The microcomputer 54 controls the mechanisms of 
the reticle inspecting apparatus and serves also as an interface between 
the operator and the reticle inspecting apparatus. 
The results of inspection are displayed, and the position of the detected 
defect determined on the basis of the results of inspection are 
transferred to an observation unit to enable the operator to recognize the 
defect. Since the reticle, i.e., a photomask, is an original plate for 
fabricating LSI circuits, there must not be even a single defect, such as 
a foreign particle, that will adversely affect correct printing of an 
image of the circuit pattern of the reticle on a wafer by photographic 
processes on the reticle. Therefore, it is important to enable the 
operator to decide whether or not the detected defect, such as a foreign 
particle, affects the printing of an image of the circuit pattern on a 
wafer adversely. The results of inspection is transferred to the 
observation unit to enable the operator to examine the results of 
inspection. A reticle inspecting apparatus shown in FIG. 62 is provided 
with an observation unit. In this reticle inspecting apparatus, the 
optical path of a detection optical system is changed to give optical 
information for examination to the observation unit. When this reticle 
inspecting apparatus is used, any separate observation apparatus is not 
necessary, the reticle can accurately and efficiently be examined, and the 
contamination of the reticle during transportation can be obviated because 
the reticle need not be transported to a separate observation apparatus. 
In FIG. 62, a signal processing system and illuminating units for 
inspection are omitted. An observation illuminating system for observation 
comprises a transmission illuminating unit 6221 provided with a shutter 
6222, a top illuminating unit 6211 having a half mirror 6212 and a driving 
mechanism 6213 for driving the half mirror 6212, and an oblique 
illuminating unit 6231 provided with a laser light source. The oblique 
illuminating unit 6231 may be omitted when the front illuminating units 
for inspection, i.e., the front illuminating units 2 and 20 in FIG. 1, 
project laser beams having wavelengths in those of visible radiation, and 
either the first front illuminating unit or the second front illuminating 
unit may be used for observation. These observation illuminating units are 
used selectively or in combination. Light reflected from a defect, such as 
a foreign particle, is concentrated by an objective lens 41 and is 
reflected by a reflecting mirror 6202 located at the working position by 
the driving mechanism 6202 to form an image by an observation device 6201, 
such as a TV camera, for observation. When necessary, a spatial filter 
6232 is inserted in the optical path by a driving mechanism 6233 during 
observation. 
Also shown in FIG. 62 is a pellicle inspecting unit 6251. Since sensitivity 
for the inspection of the pellicle and the back surface of the reticle not 
provided with the circuit pattern need not be as high as that for the 
inspection of the front surface of the reticle on which the circuit 
pattern is formed, the use of a low-sensitivity, simple, quick-operating 
inspection unit for the inspection of the reticle and the back surface of 
the reticle curtails the time necessary for inspection and simplifies the 
construction of the reticle inspecting apparatus. The reticle inspecting 
apparatus may be provided with a substrate inspecting unit having a simple 
configuration for inspecting a substrate having mirror-finished surfaces 
before forming a circuit pattern thereon, such as a glass plate or a glass 
substrate having only a surface coated with a metal thin film, because the 
surfaces not provided with any circuit pattern can be inspected for 
defects quickly in a high sensitivity by a substrate inspecting unit 
having a simple configuration. 
The operation of the reticle inspecting apparatus will be described with 
reference to FIGS. 4 to 10, in which parts like or corresponding to those 
shown in FIG. 1 are denoted by the same reference characters. FIG. 2 is a 
view of assistance in explaining a reticle scanning method, FIG. 5 is a 
plan view of assistance in explaining an angular portion of the circuit 
pattern, FIGS. 6(a)-(c) are views showing the distribution of scattered 
light and that of diffracted light on the Fourier transform plane, FIG. 
7(A) is a fragmentary plan view of a corner of a circuit pattern and FIG. 
7(B) is an enlarged view of a portion CO of FIG. 7(A), FIG. 8 is a graph 
of assistance in explaining the relation between a scattered light 
detection signal provided when scattered light scattered by a foreign 
particle is detected and a detection signal provided when a circuit 
pattern is detected, FIG. 9 is a plan view of a minute circuit pattern, 
and FIG. 10 is a graph showing the levels of detection signals provided 
when a foreign particle and corners of the circuit pattern are detected. 
FIG. 11 is a graph showing the relation between the theoretical intensity 
of scatted light scattered by particles and a non-dimensional value 
.phi.d/.lambda., where d is the size of the particle and .lambda. is the 
wavelength of the illuminating light beam. FIGS. 12(A) and 12(B) are vies 
of assistance in explaining a method of detecting scatted light from a 
foreign particle by using an optical system having a high NA in accordance 
with the present invention. FIG. 13 is a diagrammatic view showing the 
direction of travel of diffracted light 2222 and 2223 from illumination of 
light 2221 applied to the defect 70 such as a foreign particle on the 
reticle 6. FIG. 14 is a diagrammatic view of assistance in explaining the 
definition of the NA of the optical system utilizing an objective lens 41. 
FIG. 15 is a graph showing the relation between the sectional area of the 
scattered light proportional to the intensity of the scattered light 
scattered by foreign particle and the diameter d of the foreign particle. 
Shown in FIG. 4(A) are a foreign particle 70 on the reticle 6 fastened to 
the Z-stage 10 by the fastening device 18, a straight portion 81 of a 
circuit pattern 80, and a corner 82 of the circuit pattern 80. 
The reticle 6 is illuminated obliquely by the illuminating unit 2 (or any 
one of the illuminating units 20, 3 and 30). Directly reflected light and 
directly transmitted light are not gathered. Only scattered light and 
diffracted light are gathered by the objective lens 41. Only the 
diffracted light diffracted by an edge of the circuit pattern 80 extending 
at an angle .theta.=0.degree. to a direction perpendicular to the 
horizontal component 60 of the direction of travel of the illuminating 
light emitted by the illuminating unit 2 (or any one of the illuminating 
units 20, 3 and 30), which is called a 0-degree edge, is focused in a band 
as shown in FIG. 6(a) on the Fourier transform plane of the objective lens 
41. The angle .theta. of the edges of the circuit pattern 80 is 0.degree., 
45.degree. or 90.degree.. Diffracted light (b) diffracted by a 45-degree 
edge and diffracted light (c) diffracted by a 90-degree edge do not fall 
on the objective lens 41 as shown in FIG. 4(A) and do not affect the 
inspection of the reticle. Scattered light scattered by the foreign matter 
70 is scattered over the entire area of the Fourier transform plane as 
shown in FIG. 6(c). Therefore, the foreign matter 70 can be discriminated 
from the circuit pattern 80 by intercepting the diffracted light (a) 
diffracted by the 0-degree pattern shown in FIG. 4(A) by the spatial 
filters 44 and 44 disposed on the Fourier transform planes and each having 
a band-shaped screening portion and light-transmissive portions on the 
opposite sides of the opaque portion. 
Since Fourier transform planes exist behind the objective lens 41 and on 
the plane of the entrance pupil of the objective lens 41 as shown in FIG. 
4(A), the spatial filter may be disposed immediately before the objective 
lens. In this arrangement, aberration attributable to the different 
wavelengths of the component of the inspecting light beam does not occur 
because the inspecting light beam is not transmitted by the lens system 
and hence Fourier transform planes for all the wavelength coincide. 
The defect is not detected directly on the Fourier transform plane because 
the defect can be detected in a high sensitivity when the defect is 
detected on an image plane determined through the inverse Fourier 
transformation of a Fourier transform image in a reduced inspection field. 
However, since inverse Fourier transformation is a mathematical operation, 
the detection may be made by directly determining the amplitude and the 
phase difference of a Fourier transform image on the Fourier transform 
plane and executing an inverse Fourier operation by a computer, which 
increases the degree of freedom of space filtering. 
Thus, this detection optical system 4 has a large NA. When NA=0.5, the 
aperture area of the detection optical system 4 is about twenty times the 
aperture area of the conventional detection optical system having a small 
NA (NA=0.1). Scattered light scattered by a corner portion (FIG. 4(D)) of 
the circuit pattern 80 cannot completely be intercepted by the linear 
spatial filter. Therefore, when 10.times.20 .mu.m.sup.2 detecting pixels 
are used for detection (FIG. 4(B)), scattered light scattered by a 
plurality of corner portions fall on the pixels and the detection of only 
the foreign particle is impossible. 
Accordingly, the present invention uses 2.times.2 .mu.m.sup.2 pixels for 
higher resolution (FIG. 4(C)) to eliminate the influence of the scattered 
light and diffracted light scattered and diffracted, respectively, by the 
circuit pattern 80 as perfectly as possible. The size of the pixels need 
not necessarily 2.times.2 .mu.m.sup.2. The pixels may be of any size, 
provided that the size is smaller than the size L of the smallest portion 
of the circuit pattern 80. When the reticle is exposed by a stepper having 
a reduction ratio of 1/5 when fabricating a 0.8 .mu.m process LSI, pixels 
of 0.8.times.5=4 .mu.m.sup.2 or below serve the purpose, and pixels of 
0.5.times.5=2.5 .mu.m.sup.2 or below serve the purpose when fabricating a 
0.5 .mu.m process LSI. 
Practically, the size of the pixels may be greater or smaller than the 
above-mentioned sizes as long as the pixels are able to reduce the 
influence of the scattered light scattered by the corner portions of the 
circuit pattern to the negligibly small extent. Concretely, a desirable 
size of the pixels is nearly equal to the size of the smallest portion of 
the circuit pattern. When the size of the pixels is on the order of the 
size of the smallest portion of the circuit pattern, only less than two 
corner portions correspond to each pixel, and as is obvious from the 
results of experiment shown in FIG. 10, the size of the pixels is small 
enough. Pixels having a size in the range of 1 to 2 .mu.m.sup.2 are 
desirable for inspecting a reticle for fabricating a 64M DRAM. 
Since a corner 82 of the circuit pattern 80 shown in FIG. 7(A) has a 
continuously curved edge 820 as shown in FIG. 7(B), diffracted light (d) 
diffracted by the corner 82 is scattered on the Fourier transform planes 
as shown in FIG. 6(b) and the spatial filters 44 and 444 are unable to 
intercept the diffracted light (d) completely. Consequently, the output V 
of the detector 51 (551) increases as shown in FIG. 8 and the foreign 
matter 70 cannot be discriminated from the circuit pattern 80 if 
diffracted light diffracted by a plurality of corners 82 falls on the 
detector 51 (551). As shown in FIG. 8, the output 822 of the detector 51 
(551) provided when a plurality of corners 82 are detected is higher than 
the output 821 of the same provided when a single corner 82 is detected. 
If the output of the detector 51 (551) is binarized by using a binarizing 
threshold 90 indicated by a dotted line, the output 701 of the detector 51 
(551) representing the foreign particle 70 cannot be discriminated from 
the output 822 of the detector 51 (551) representing the plurality of 
corners 82. 
To solve the problem described with reference to FIG. 8, the present 
invention forms an image of the inspection field 15 on the detectors 51 
and 551 by means of the objective lens 41 and the focusing lenses 45, 
determines the sizes of the detectors 51 and 551 and image forming 
magnification selectively to determine the size of the inspection field 15 
(for example, 2 .mu.m.times.2 .mu.m) optionally in order to obviate the 
simultaneous incidence of diffracted light diffracted by a plurality of 
corners 82 on the detectors 51 and 551. However, this arrangement is not 
sufficiently effective to discriminate a foreign particle of a size on the 
submicron order from a corner 82 of the circuit pattern 80. Furthermore, 
since the behavior of diffracted light diffracted by a portion of a size 
84 on the submicron order, which is smaller than the size 83 of other 
portions of the circuit pattern 80 as shown in FIG. 9, is similar to that 
of scattered light scattered by the foreign particle 70, it is difficult 
to discriminate the foreign particle 70 from such a minute circuit 
pattern. 
The reticle inspecting apparatus of the present invention is capable of 
detecting the foreign particle 70 in such a minute circuit pattern having 
portions of a size 84 on the submicron order. In FIG. 10, 701 and 702 
indicate a detection signal provided upon the detection of scattered light 
scattered by a minute foreign particle 70 of a size on the submicron 
order, 864, 874, 865, 875, 866, 876, 867 and 877 indicate detection 
signals provided upon the detection of scattered light scattered by all 
the corners 82 of the 0-degree, 45-degree and 90-degree edges, and 861, 
871, 862, 872, 863 and 873 indicate detection signals provided upon the 
detection of scattered light scattered by minute portions of sizes 84 on 
the submicron order. The detection signals 701, 861, 862, 863, 864, 865, 
866 and 867 are provided by the detector when the illuminating light beam 
projected by the first front illuminating unit 2 (or the first back 
illuminating unit 3) and scattered by the minute circuit pattern is 
detected, and the detection signals 702, 871, 872, 873, 874, 875, 876 and 
877 are provided by the detector when the illuminating light beam 
projected by the second front illuminating unit 20 (or the second back 
illuminating unit 30) and scattered by the minute circuit patter is 
detected. For example, the detection signals 861.rarw..fwdarw.871 are 
those provided when the illuminating light beam projected by the first 
front illuminating unit 2 (or the first back illuminating unit 3) and 
scattered by a portion of the minute circuit pattern is detected by the 
detector and when the illuminating light beam projected by the second 
front illuminating unit 20 (or the second back illuminating unit 30) and 
scattered by the same portion of the minute circuit pattern is detected by 
the detector, respectively. As is obvious from FIG. 10, the value of the 
detection signal provided upon the detection of the foreign particle 70 is 
less dependent on the direction of projection of the illuminating light 
beam than that of the detection signal provided upon the detection of a 
portion of the minute circuit pattern. In FIG. 10, a dotted line 91 
represents the threshold for binarization. 
As is obvious from FIG. 10, the value of the detection signal provided by 
the detector upon the detection of a portion of the minute circuit pattern 
is greatly dependent on the direction of projection of the illuminating 
light beam and, when a portion of the surface of the reticle 6 is 
illuminated obliquely by two illuminating light beams which travel 
respectively along paths which are symmetrical with respect to a normal to 
the surface of the reticle 6 at the illuminated portion, either of the 
detection signals provided upon the detection of the two illuminating 
light beams scattered by the illuminated portion is necessarily smaller 
than the detection signal provided upon the detection of the scattered 
light scattered by the foreign particle of a size on the submicron order 
as indicated by solid circles. When the illuminating light beams are 
projected obliquely by both the first front illuminating unit 2 and the 
second front illuminating unit 20 which are disposed symmetrically with 
respect to a normal to the surface of the reticle 6 at the illuminated 
position or by both the first back illuminating unit 3 and the second back 
illuminating unit 30 which are disposed symmetrically with respect to a 
normal to the surface of the reticle 6 at the illuminated position, the 
detection signal is the sum of the detection signal provided upon the 
detection of the scattered illuminating beam projected by one of the two 
front (back) illuminating units and scattered by the foreign particle or a 
portion of the circuit pattern and the detection signal provided upon the 
detection of the scattered illuminating beam projected by the other front 
(back) illuminating unit and scattered by the same foreign particle or the 
same portion of the circuit pattern. Thus, the foreign particle can be 
illuminated in an illuminance higher than an illuminance in which the 
circuit pattern is illuminated, whereby the foreign particle 70 of a size 
on the submicron order can be discriminated from the circuit pattern 80. 
When the scattered light scattered by the foreign particle 70 is detected, 
the microcomputer 54 stores foreign particle data including information 
about the respective positions of the X-stage 10 and the Y-stage 11, 
information about the position of the foreign particle 70 calculated on 
the basis of the position of the corresponding pixel, and the detection 
signals of the detectors 51 and 551 in a storage device and displays the 
foreign particle data on the display 55, such as a CRT. 
As explained above with reference to FIG. 10, the value of the detection 
signal provided by the detector upon the detection of a portion of the 
circuit is greatly dependent on the direction of projection of the 
illuminating light beam and, when a portion of the surface of the reticle 
6 is illuminated obliquely by two illuminating light beams which travel 
respectively in opposite directions, either of the detection signals 
provided upon the detection of the two illuminating light beams scattered 
by the illuminated position is always smaller than the detection signal 
provided upon the detection of a defect, such as a foreign particle as 
indicated by solid circles in FIG. 10. 
Accordingly, when the surface of the reticle 6 is illuminated obliquely by 
two illuminating light beams traveling respectively in two opposite 
directions, each of the detection signals provided upon the detection of a 
foreign particle and the circuit pattern is merely the sum of the 
detection signals and it is difficult to binarize the detection signal by 
the threshold. However, when the scattered opposite illuminating light 
beams are detected by two separate detectors and the respective output 
signals of the two detectors are binarized by the threshold 91, both the 
outputs of the binarizing circuit are "1" when the illuminating light 
beams scattered by a foreign particle are detected, and both the outputs 
of the binarizing circuits are "0" or one of the outputs of the same is 
"1" when the illuminating light beams scattered by the circuit patter are 
detected. Thus, the foreign particle 70 of a size on the submicron order 
can be discriminated from the circuit pattern by determining the logical 
product of the outputs of the binarizing circuits through logic operation. 
The scattered illuminating light beams projected respectively along 
opposite directions must be detected separately for the detection of a 
foreign particle by means of the AND circuit. The front illuminating units 
2 and 20 are provided respectively with light sources capable of emitting 
illuminating light beams differing from each other in wavelength, 
respectively, to enable wavelength separation (color separation) or of 
emitting illuminating light beams differing from each other in 
polarization characteristic, respectively, to enable polarization 
separation for the discrimination of a foreign particle from the circuit 
pattern by using the AND circuit. Since the illuminating light beams are 
intercepted by the frame holding the pellicle, the entire front surface of 
the reticle cannot be illuminated with illuminating light beams traveling 
respectively in opposite directions by the front illuminating units 2 and 
20; that is, the areas 3724 and 3704 in FIG. 37 cannot be illuminated with 
illuminating light beams traveling respectively in opposite directions. 
The present invention utilizes the following facts: (1) a high sensitivity 
for detection is necessary for the detection of foreign particles in 
light-transmissive areas, i.e., exposed portions of the surface of the 
glass substrate, in the surface of the reticle because the output signal 
of the detector representing the intensity of scattered light scattered by 
a foreign particle in a light-transmissive area is low, and the 
calculation of logical product is not necessary for the detection of a 
foreign particle on the opaque, metal thin film, such as a chromium film, 
because the output signal of the detector representing the intensity of 
scattered light scattered by a foreign particle on the opaque, metal thin 
film is high; (2) the light sources of the front illuminating units and 
those of the back illuminating units are different in wavelength from each 
other; and (3) the entire surface of the reticle can be illuminated 
without being obstructed by the frame holding the pellicle by using the 
front illuminating unit and the back illuminating unit, which will be 
described later. 
The present invention illuminates a reticle to detect a foreign particle in 
a light-transmissive area on the reticle by the first front illuminating 
unit 2 and the second back illuminating unit 30 or by the second front 
illuminating unit 20 and the first back illuminating unit 3, separates the 
scattered light beams by wavelength separation, and binarizes the output 
signals of the detectors. 
FIGS. 71(A)-71(C) and 72(A)-72(C) are sectional views of assistance in 
explaining the effect. Shown in FIGS. 71(A)-71(C) and 72(A)-72(C) are a 
glass substrate 6901, a front illuminating light beam 6904 having a 
wavelength .lambda.1 for obliquely illuminating the circuit pattern formed 
on the front surface of the glass substrate 6901, a back illuminating 
light beam 6905 having a wavelength .lambda.2 and traveling in a direction 
opposite the direction in which the front illuminating light beam 6904 
travels, for illuminating the circuit pattern from the side of the back 
surface of the glass substrate 6901, edges 6902 and 7002 of the circuit 
pattern, scattered front illuminating light 6942, which is part of the 
front illuminating light beam 6904 scattered by the edge 6902 of the 
circuit pattern, scattered back illuminating light 6952, which is part of 
the back illuminating light beam 6905 scattered by the edge 6902 of the 
circuit pattern, scattered front illuminating light 7042, which is part of 
the front illuminating light beam 6904 scattered by the edge 7002 of the 
circuit pattern, scattered back illuminating light 7052, which is part of 
the back illuminating light beam 6905 scattered by the edge 7002 of the 
circuit pattern, standard particles 6903 and 7003 of a size on the order 
of 0.3 .mu.m, i.e., typical models of foreign particles, scattered front 
illuminating light 6943, which is part of the front illuminating light 
beam 6904 scattered by the standard particle 6903, scattered back 
illuminating light 6953, which is part of the back illuminating light beam 
6905 scattered by the standard particle 6903, scattered front illuminating 
light 7043, which is part of the front illuminating light beam 6904 
scattered by the standard particle 7003, and scattered back illuminating 
light 7053, which is part of the back illuminating light beam 6905 
scattered by the standard particle 7003. 
The intensity of the scattered light is greatly dependent on the incidence 
angle of the illuminating light beam on the circuit pattern when the 
illuminating light beam is projected obliquely on the circuit pattern 
having a thickness, which is very small though. For example, in FIG. 71, 
the intensity of the scattered back illuminating light is comparatively 
high and the intensity of the scattered front illuminating light is 
comparatively low. The intensities of the scattered front illuminating 
light and the scattered back illuminating light scattered by a minute 
foreign particle not having distinct anisotropy are not very different 
from each other. 
As shown in graphs of FIG. 71 showing detection signals (V) corresponding 
to the intensities of the different kinds of scattered illuminating light, 
the intensity of the scattered back illuminating light 6952 scattered by 
the edge of the circuit pattern is higher than that of the scattered back 
illuminating light 6953 scattered by the standard particle 6903. If a 
threshold Th2 for simple binarization is used, a foreign particle cannot 
be discriminated from the edge of the circuit pattern. However, the 
intensity of the scattered front illuminating light 6943 scattered by the 
standard particle is higher than that of the scattered front illuminating 
light 6942 scattered by the edge of the circuit pattern and hence a 
foreign particle can be discriminated from the edge of the circuit pattern 
by using the threshold Th1 for simple binarization. When the circuit 
pattern has the edge 6902 as shown in FIG. 71, the scattered front 
illuminating light 6942, i.e., part of the front illuminating light beam 
6904 scattered by the edge 6902, may be detected. FIG. 72 shows the edge 
7002 facing in a direction different from that in which the edge 6902 
faces relative to the direction of the front illuminating light beam 6904. 
As shown in FIG. 72, the intensity of the scattered front illuminating 
light 7042, i.e., part of the front illuminating light beam 6904 scattered 
by the edge 7002, is comparatively high and the intensity of the scattered 
back illuminating light 7052, i.e., part of the back illuminating light 
beam 6905 scattered by the edge 7002, is comparatively low, and the 
difference between the intensities of the scattered front illuminating 
light 7043 and the scattered back illuminating light 7053 scattered by a 
minute particle not having distinct anisotropy is not very large. 
As shown in FIG. 72, the intensity of the scattered front illuminating 
light 7042 scattered by the edge 7002 is higher than that of the scattered 
front illuminating light 7043 scattered by the standard particle. 
Therefore, if the threshold Th1 for simple binarization is used, a foreign 
particle cannot be discriminated from the edge. On the other hand, the 
intensity of the scattered back illuminating light beam 7053 scattered by 
the standard particle is higher than that of the scattered back 
illuminating light 7052 scattered by the edge 7002 of the circuit pattern, 
and a foreign particle can be discriminated from the edge of the circuit 
pattern by using the threshold Th2 for simple binarization. When edges of 
the circuit pattern face in the same direction as that in which the edge 
7002 shown in FIG. 72 faces, foreign particles can be discriminated from 
the edges by detecting the scattered back illuminating light, i.e., part 
of the back illuminating light beam 6905. However, an actual circuit 
pattern has both edges like the edge 6902 shown in FIG. 71 and edges like 
the edge 7002 shown in FIG. 2, and hence it is ineffective to detect 
either the scattered front illuminating light or the scattered back 
illuminating light selectively. When the reticle is inspected by a reticle 
inspecting method in accordance with the present invention, the 
intensities of the scattered front illuminating light, i.e., part of the 
front illuminating light beam 6904 and the scattered back illuminating 
light, i.e., part of the back illuminating light beam 6905 are higher than 
both the threshold Th1 and Th2 for binarization, and the intensities of 
both the scattered front illuminating light beam and the scattered back 
illuminating light scattered by the edges (FIGS. 71 and 72) are always 
lower than both the threshold Th1 and Th2 for binarization. Therefore, 
only the scattered light scattered by foreign particles can be detected by 
detecting the intensities of the scattered front illuminating light, i.e., 
part of the front illuminating light beam 6904, and the back illuminating 
light, i.e., part of the back illuminating light beam 6905, binarizing the 
intensities by using the threshold Th1 and Th2 for binarization, and 
carrying out an AND operation between the binarized intensities. 
The scattered front illuminating light and the scattered back illuminating 
light can easily be separated with a color separation filter or the like 
and the front illuminating light beam 6904 and the back illuminating light 
beam 6905 can be used simultaneously, when the respective wavelengths of 
the front illuminating light beam 6904 and the back illuminating light 
beam 6905 are different from each other, which enables real-time reticle 
inspection. 
FIGS. 39(A) and 39(B), similarly to FIGS. 38(A) to 38(C), shows the 
positional relation between the reticle 6, the pellicle 7, the oblique 
illuminating light beam 3802 projected by the first front illuminating 
unit 2, the oblique illuminating light beam 3820 projected by the second 
illuminating unit 20, the oblique illuminating light beam 3803 projected 
by the first back illuminating unit 3, the oblique illuminating light beam 
3830 projected by the second back illuminating unit 30 (FIG. 1) and the 
inspection field 15, i.e., the illuminated position. FIG. 39(A) shows a 
first illuminating mode for inspecting an area 4024 on the left side of 
the center line 4001 of the frame holding the pellicle 7 (FIG. 40) by 
using the oblique illuminating light beams 3820 and 3803 projected 
respectively by the second front illuminating unit 20 and the first back 
illuminating unit 3. FIG. 49(B) shows a second illuminating mode for 
inspecting an area 4004 on the right side of the center line 4001 by using 
the oblique illuminating light beams 3802 and 3830 projected respectively 
the the first front illuminating unit 2 and the second back illuminating 
unit 30. These illuminating modes are used to prevent the interception of 
the illuminating light beam by the frame 3807 and the first illuminating 
mode need not necessarily be changed for the second illuminating mode when 
the inspection field 15 moves across the center line 4001. 
FIG. 41 is a block diagram of part of the signal processing system 5 of 
FIG. 1 for processing the detection signals provided by the detectors in 
the reticle inspecting operation in the aforesaid illuminating modes. In 
FIG. 41, parts corresponding to those shown in FIG. 1 are denoted by the 
same reference characters. 
Referring to FIG. 41, scattered back illuminating light, i.e., part of the 
illuminating light beam emitted by the first back illuminating unit 3, not 
shown, or the second back illuminating unit 30, not shown, travels through 
the wavelength separation mirror 42, not shown, and falls on the detector 
51, while scattered front illuminating light beam, i.e., part of the 
illuminating light beam projected by the first front illuminating unit 2, 
not shown, or the second front illuminating unit 20, not shown, is 
reflected by the wavelength separation mirror 42 and falls on the detector 
551. 
The binarizing circuits 52 and 552 receive the respective detection signals 
4101 and 4111 of the detectors 51 and 551, execute logic operation to 
convert the detection signals 4101 and 4111 into corresponding logical 
values 4103 and 4113, respectively, and give the logical values 4103 and 
4113 to the AND circuit 57. The AND circuit 57 carries out the logical AND 
operation between the logical values 4103 and 4113 to provide the logical 
product of the logical values 4103 and 4113. The output 4102, i.e., the 
logical product, is the results of detection of a defect, such as a 
foreign particle. As mentioned, it is desirable to provide the detection 
signals in addition to the logical level of the output 4102 of the AND 
circuit 57, which applies also to the following cases. 
Referring to FIG. 69 showing a configuration for such a case, a threshold 
6702 for binarizing detection signals provided upon the detection of 
scattered back illuminating light (back illuminating light detection) is 
set beforehand for a binarizing circuit 6701, and a threshold 6701 for 
binarizing detection signals provided upon the detection of scattered 
front illuminating light (front illuminating light detection) is set 
beforehand for a binarizing circuit 6701. A detection signal 6703 obtained 
by back illuminating light detection is given to the binarizing circuit 
6701, and a detection signal 6713 obtained by front illuminating light 
detection is given to the binarizing circuit 6711. An AND device 6721 
carries out an AND operation between the outputs of the binarizing 
circuits 6701 and 6711. When the output of the AND device 6721 is logical 
"1", a data selector 6731 selects the detection signal provided by back 
illuminating light detection and provides an output 6732. The output 6732 
may be equal to the detection signal obtained by back illuminating light 
detection. 
Although the foregoing procedure is effective on detecting foreign 
particles in the transparent area of the photomask, such as a reticle, the 
procedure entails the following problems in detecting foreign particles in 
the opaque area of the photomask. Since a foreign particle in the opaque 
area cannot be illuminated with the back illuminating light beam, the 
foreign particle does not scatter the back illuminating light beam. For 
example, when there is a large foreign particle, which must be detected, 
in the opaque area, the front illuminating light beam is scattered by the 
foreign particle and scattered front illuminating light having a high 
intensity is detected while the back illuminating light beam is not 
scattered and, consequently, the output of the binarizing circuit 6701 is 
logical "0", while the output of the binarizing circuit 6711 is logical 
"1". Accordingly, even if the output of the binarizing circuit 6711 (FIG. 
69) is logical "1", it is not decided that there is a large foreign 
particle on the reticle. 
To prevent such erroneous foreign particle detection, it is necessary to 
decide that there is a foreign particle on the reticle when the intensity 
of the scattered front illuminating light is high, even if no scattered 
back illuminating light is detected. Although this signal processing 
procedure is incapable of detecting minute foreign particles, scattered 
front illuminating light beam scattered by which has a low intensity, in 
the opaque area, an image of such a minute foreign particle in the opaque 
area is not printed on the wafer as mentioned above, and only large 
foreign particles which are liable to migrate from the opaque area to the 
transparent area need to be detected. Therefore, this signal processing 
procedure does not cause practical problems. 
FIG. 74 shows a signal processing system capable of carrying out such a 
signal processing procedure. The signal processing system shown in FIG. 74 
is provided, in addition to the components of the signal processing system 
of FIG. 69, a binarizing circuit 7201 for binarizing a detection signal 
provided upon the detection of scattered front illuminating light, and an 
OR circuit 7221. A threshold 7212 is set beforehand for the binarizing 
circuit 7201. The threshold 7212 is greater than the threshold 6712; that 
is the threshold 7212 is greater than the level of the detection signal 
representing the intensity of scattered light scattered by the circuit 
pattern. When a detection signal of a level exceeding the threshold 7212 
is provided, the output of the binarizing circuit 7201 is logical "1". The 
OR circuit 7221 carries out an OR operation between the output of the 
binarizing circuit 7201 and the output of the AND circuit 6721. When the 
output of the OR circuit 7221 is logical "1", the data selector 6731 
provides an output 6732 corresponding to a detection signal 6732 obtained 
by front illuminating light detection. In this case, only the detection 
signal obtained by front illuminating light detection is provided because 
foreign particles in the opaque area cannot be illuminated with the back 
illuminating light beam. 
In the AND circuit 57 or the peripheral of the AND circuit 57 formed in a 
configuration as shown in FIG. 46, the result of the logical AND operation 
appears at the output 4102 of the AND circuit 57 when a detection signal 
4103 is applied through a switching device 4133 to one of the inputs of 
the AND circuit 4157, and results of detection of a detection system not 
using an AND system appear at the outputs 4102 and 4112 of the AND circuit 
57 when an input 4123 of logical "1" is applied through the switching 
device 4133 to the same input of the AND circuit 57. Thus, the detection 
mode of the reticle inspecting apparatus can selectively be determined. In 
such a case, a circuit shown in FIG. 47 is used instead of the circuit 
shown in FIG. 41. Since the purpose of use of the circuit shown in FIG. 46 
is to enable the selective determination of the detection mode of the 
reticle inspecting apparatus, suitable software or the like may be 
employed instead of the circuit shown in FIG. 47. 
When it is decided that scattered light scattered by a defect, such as a 
foreign particle 70, is detected, defect data indicating the positions of 
the X-stage 10 and the Y-stage 11 at the moment of detection of the 
foreign particle 70, the position of the foreign particle 70 calculated on 
the basis of the positions of the pixels received the scattered light 
among those of the detectors 51 and 651, and the respective detection 
signals 4101 and 4111 provided by the detectors 51 and 551 is stored in a 
storage device controlled by the microcomputer 54, the contents of the 
storage device are processed and the processed contents of the storage 
device are displayed on the display 55, such as a CRT. 
Detection and identification of a foreign particle on the basis of the 
output of each pixel of a detector of an array type entails the following 
problems. 
Suppose that a detector having pixels of 2.times.2 .mu.m.sup.2 is used for 
the detection and identification of a foreign particle. Then, if the 
foreign particle is detected by four pixels as shown in FIG. 26, the 
scattered light scattered by the foreign particle is distributed to the 
plurality of pixels and the output of each pixel is in the range of 1/2 to 
1/4 (practically about 1/3 due to crosstalk between the pixels) of the 
output which may be obtained when the foreign particle is detected by a 
single pixel and, consequently, the detection probability is reduced. 
Furthermore, the positional relation between the pixels of the detector 
and the minute foreign particle is delicate and very liable to change and 
changes every time the inspection is made, which deteriorate the 
repeatability of the inspection. Such problems arise also when the foreign 
particle is detected by two pixels or three pixels as well as when the 
foreign particle is detected by four pixels. 
To overcome the foregoing disadvantage, 1.times.1 .mu.m.sup.2 pixels as 
shown in FIG. 27 are used, and the respective detection signals provided 
by the four adjacent 1.times.1 .mu.m.sup.2 pixels are added electrically 
to simulate the detection signal of a 2.times.2 .mu.m.sup.2 pixel. The 
sums of the detection signals each of four adjacent pixels of each of four 
duplicate pixel groups a, b, c and d are calculated, and the maximum sum 
of outputs, i.e., the sum of outputs of the pixels of the pixel group a in 
FIG. 27, is considered to be equivalent to the output of a 2.times.2 
.mu.m.sup.2 pixel and used as a foreign particle detection signal. In this 
method of detecting a foreign particle, the variation of the detection 
signal indicating a foreign particle is within .+-.10% and the 
repeatability of detection for all the foreign particles is 80% or above. 
FIG. 28 is a block diagram of a four-pixel addition circuit. This 
four-pixel addition circuit is used in combination with a one-dimensional 
imaging device provided with an array of 512 l .mu.m.sup.2 pixels, which 
provides the output 2502 of even-numbered pixels and the output 2503 of 
odd-numbered pixels separately. The outputs of four 1.times.1 .mu.m.sup.2 
pixels (2.times.2 pixels) shifted by one pixel in four directions are 
added by 256-stage shift registers 2501, one-stage shift registers 2504 
and an adders 2505 to 2508, and dividers 2509 to 2512 calculate the means 
of the outputs of the pixels. A maximum value selecting circuit 2513 
selects the maximum mean as a foreign particle detection signal 2514 from 
among the four means. 
When the four-pixel addition circuit shown in FIG. 28 is used, one 
detection signal is provided for a unit of 2 .mu.m and the quantity of 
data is reduced by a factor of four. Therefore, the signal processing 
speed of the subsequent signal processing circuit may be reduced by a 
factor of four, which is advantageous to designing the circuits and to the 
operation of the circuits. Thus, defect detecting operation can stably be 
carried out. 
The foregoing operations for adding the outputs of the adjacent four pixels 
and calculating the means are intended to prevent the reduction of the 
level of detection signals provided by four pixels corresponding to one 
foreign particle. Therefore, each pixel group may have more than four 
pixels or less than four pixels, provided that the desired object can be 
achieved. FIG. 67 shows an example of two-pixel addition operation. As 
shown in FIG. 67, rectangular pixels are used instead of square pixels. 
This can be realized by using a rectangular detector or by moving the 
stage at a comparatively high feed speed as compared with the detecting 
time of the detector; for example, a detector having a size of 1 
.mu.m.times.1 .mu.m on the reticle is used and the stage is moved 2 .mu.m 
in detecting time T to form 1 .mu.m.times.2 .mu.m pixels on the reticle. 
As shown in FIG. 67, the detection signals of two pixels are added. 
In the embodiment shown in FIG. 67, (a1+a2)/2, (a2+a3)/2, (b1+b2)/2, 
(b2+b3)/2, (a1+b1)/2, (b1+c1)/2, (a2+b2)/2 and (b2+c2)/2 are calculated at 
time b2, and the maximum among the calculated results, i.e., the maximum 
mean among the means of added values, is provided as the detection signal. 
Although the effect of the two-pixel addition method on preventing the 
reduction of the level of the detection signals provided upon the 
detection of a foreign particle corresponding to four pixels is lower than 
that of the four-pixel addition method, the stage feed speed of the 
two-pixel addition method is higher than that of the four-pixel addition 
method and hence the two-stage addition method enhances the speed of 
inspection. 
In the foregoing embodiment, the size of the foreign particle to be 
detected, for example, 0.5 .mu.m, is smaller than the size of the pixels, 
for example, 2 .mu.m.times.2 .mu.m, of the detector. Therefore, if a 
foreign particle corresponds to one pixel of, for example, 1 .mu.m.times.1 
.mu.m, a detection signal provided by the detector before the four-pixel 
addition operation is equal to a detection signal obtained by the 
four-pixel addition operation and provided by the detector, because the 
detection signal compensating effect of the four-pixel addition operation 
is effective only when a foreign particle corresponds to a plurality of 
pixels. The smaller the area (size) of the pixels of the detector, the 
smaller is the number (area) of the corners of the circuit pattern 
corresponding to one pixel and hence the lower is the intensity of 
scattered light scattered by the circuit pattern. Therefore, the smaller 
pixels are desirable for for the detection of foreign particles in higher 
sensitivity. It may safely be said that the four-pixel addition method 
sacrifices detection sensitivity for stable detection. Although any 
measures need not be taken to solve this problem if the reduced detection 
sensitivity is high enough, some measures must be taken to make the 
detection sensitivity effective even if process conditions and exposure 
method are changed. 
The problem may be solved by selectively using a high-stability detection 
mode including the four-pixel addition operation or a high-sensitivity 
detection mode not including the four-pixel addition operation according 
to required performance. 
Both the purposes of high-stability detection mode and the high-sensitivity 
detection mode can be achieved through the detection of foreign particles 
before and after the four-pixel operation. The present invention employs a 
signal processing system as shown in FIG. 42 to achieve the purposes of 
both the high-stability detection mode and the high-sensitivity detection 
mode. 
Referring to FIG. 42, a detection signal provided by a detector 51 (551) is 
processed by a four-pixel addition circuit 114 (124), the output of the 
four-pixel addition circuit 114 (124) is binarized by a binarizing circuit 
52 (552), and the detection signal provided by the detector 51 (551) is 
binarized by a binarizing circuit 53 (553), the outputs of the binarizing 
circuits 114 (124) and 53 (553) are stored in a storage included in a 
computer 54, and displayed on a display 55. 
A foreign particle can be detected on the basis of either the output of the 
binarizing circuit 52 (552) or that of the binarizing circuit 53 (553). 
Therefore, the quantity of defect detection data obtained by detecting 
foreign particles on the circuit pattern can be reduced by subjecting the 
outputs of the binarizing circuits 52 and 552, and the outputs of the 
binarizing circuits 53 and 553 to the logical OR operation of OR circuits 
56 and 556 and storing the outputs of the OR circuits 56 and 556 in a 
storage included in a computer 54 as shown in FIG. 43. When the four-pixel 
addition operation provides the maximum value to reduce the quantity of 
data, it is difficult to compare the data provided by the four-pixel 
addition operation with the data not processed by the four-pixel addition 
operation. In such a case, the maximum value among the four detection 
signals not processed by the four-pixel addition operation is used to 
reduce the quantity of data by a factor of four as shown in FIG. 68 to 
facilitate the logical OR operation. The maximum among a1, a2, a3 and a4 
at a moment b2 is provided as a detection signal. 
The desirable effect of providing the detection signals in addition to the 
logical level holds good for the logical OR operation as well as for the 
logical AND operation. 
Such a method of detection can be achieved by an arrangement shown in FIG. 
70. A threshold 6802 is set for a binarizing circuit 6801 for binarizing 
detection signals not processed by the four-pixel addition operation, and 
a threshold 6812 is set for a binarizing circuit 6811 for binarizing 
detection signals obtained through the four-pixel addition operation. The 
binarizing circuit 6801 binarizes detection signals 6803 not processed by 
the four-pixel addition operation, and the binarizing circuit 6811 
binarizes detection signals obtained through the four-pixel addition 
operation. An OR circuit 6821 carries out the logical OR operation between 
the outputs of the binarizing circuits 6801 and 6811. When the output of 
the OR circuit 6821 is logical "1", a data selector 6831 selects a 
detection signal 6832. 
When the signal processing system provided with neither the OR circuit 56 
nor the OR circuit 556 as shown in FIG. 42 is employed, it is desirable to 
carry out the logical OR operation between the detection signals by 
software before displaying the detection signals on the display 55. 
When the logical AND operation is not used for defect detection, the 
logical OR operation between the outputs of the binarizing circuits 52 and 
552 and between the outputs of the binarizing circuits 53 and 553 may be 
carried out by the OR circuit 5556 as shown in FIG. 44 or by software. 
As shown in FIG. 45, when the logical AND operation is used for defect 
detection, an AND circuit 57 carries out the logical AND operation between 
the outputs of OR circuits 56 and 556. Since the result of the logical AND 
operation is the final result of defect detection, it is desirable to 
carry out the logical AND operation during inspection. The execution of 
the logical AND operation by an AND circuit is more practical than that by 
software. 
The reticle inspecting apparatus of the present invention actualizes only 
foreign particles optically for detection, and binarizes the detection 
signal when the detection signal is greater than the threshold to detect a 
foreign particle. However, the detection signal is subject to change due 
to (1) the difference between the pixels in sensitivity (about .+-.15%) 
and (2) the difference between the pixels in output level attributable to 
the distribution of illuminance on the reticle (shading). Therefore, the 
different pixels provides different detection signals for the same foreign 
particle as shown in FIG. 29 and the level of the output signal is 
dependent on the position of the pixel with respect to the direction along 
the Y-axis. Thus, it is impossible to detect a foreign particle stably 
through the binarization of the detection signal exceeding the threshold. 
The present invention measures the shading effect of the causes (1) and (2) 
(FIG. 30(a)) beforehand by using a standard reticle 111 (FIG. 1), 
calculates the reciprocal of the measured shading effect to determine 
shading compensating data (FIG. 30(b)) and controls the gain of an 
amplifier for amplifying the detection signal of the detector for the 
respective outputs of the pixels to obtain compensated outputs of the 
pixels (FIG. 30(c)) by eliminate the influence of the shading effect. The 
standard reticle 111 may be mounted on or disposed near the Z-stage 10 of 
the inspection stage unit 1 or may be mounted on the Z-stage only when 
measuring the shading effect. 
The standard reticle 111 has a surface having minute irregularities and 
uniform scattering characteristic. For example, the standard reticle 111 
may be a glass plate having a surface having minute irregularities formed 
by grinding, a glass plate having a surface to which standard particles of 
a specific size are attached uniformly or a plate provided with an 
aluminum film formed by sputtering. Practically, it is difficult to form 
minute irregularities corresponding to a 1.times.1 .mu.m.sup.2 pixel 
uniformly for the standard reticle 111. Therefore, the shading effect 
measurement is repeated many times, for example 1000 times and determines 
the compensated data on the basis of the mean of the measured data. 
Since only portions of the surface of the standard reticle 111 having 
minute irregularities scatter light and light is not scattered by the 
entire surface of the standard reticle 111, the addition of measured 
values obtained by repeating measurement 1000 times is not equivalent to 
and far smaller than the addition of 1000 distributions of illuminance 
over the entire illuminated area of the surface of the standard reticle 
111. Therefore the simple mean, such as the means of measured data 
obtained by dividing the sum of measured data by the number of repetition 
of measurement, is excessively small for accurate calculation. Under such 
a condition, the mean may be determined by dividing the sum of the 
measured data by a divisor, for example, 200, which is a fragment of the 
number of repetition of measurement, for example, 1000. 
As is obvious from the comparative examination of FIGS. 30(a) and 30(c), 
the shading of about 50% (FIG. 30(a) is reduced to 5% or below by 
correction. The adverse effect of the optical components of causes of 
variation in the compensated data attributable to the time-dependent 
variation of the performance of the illuminating system and the detection 
optical system can be eliminated by determining and renewing the 
compensated data every time the inspection is conducted. 
As shown in FIG. 31 a shading compensating circuit for compensating shading 
comprises a subtracter 3209 which subtracts data representing the dark 
current of each pixel and read from a memory 3206 controlled by a 
synchronizing circuit 3205 from an 8-bit value 3212 (256 steps) obtained 
through the A/D conversion of a detection signal provided by the 
one-dimensional imaging device, a multiplier 3210 which multiplies a 
shading compensation factor by data for each pixel read from a memory 3207 
controlled by a synchronizing circuit 3205, and a medium bit signal output 
circuit 3211 which changes the number of bits of a calculated 16-bit 
value, which is twice the number of bits, i.e., eight bits, of the 8-bit 
value 3212 obtained through the A/D conversion of the detection signal 
provided by the one-dimensional imaging device, for the initial number of 
bits, i.e., eight bits. Although this shading compensating circuit is a 
digital circuit that deals with digital values, analog data may be used 
for compensation. 
If 2.times.2 .mu.m.sup.2 pixels are used for detecting foreign particles 
having sizes greater than 2 .mu.m, the number of pixels detected the 
foreign particles is not equal to the number of detected foreign 
particles. When 2.times.2 .mu.m.sup.2 pixels are used for detecting A 
foreign particle of 10 .mu.m, twenty-five pixels (10.sup.2 /.sup.2 =25) 
will provide detection signals and the twenty-five detection signals must 
be examined to observe the detected foreign particle. 
The conventional reticle inspecting method examines the positional relation 
between the pixels which have detected foreign particles by software and 
decides that one foreign particle is detected by grouping when the pixels 
which have detected foreign particles are adjacent pixels to avoid the 
necessity of examining so many detection signals. This conventional 
method, however, needs software processes and need much time for 
processing many detection signals, for example, about ten minutes for 
processing 1000 detection signals. 
The present invention divides the entire inspection region into a plurality 
of field blocks (blocking) which can be simultaneously observed, such as 
field blocks each of 32.times.32 .mu.m.sup.2, and regards all the 
detection signals corresponding to each field block as detection signals 
obtained by detecting one and the same foreign particle. Thus, even a 
large foreign particle can be caught in the field block for observation 
regardless of its shape. 
Although blocking is functionally equivalent to grouping, blocking can be 
easily achieved by hardware. The present invention carries out blocking in 
a real-time mode by hardware, reduces the inspection time and enhances the 
throughput of the reticle inspecting apparatus. The reticle inspecting 
apparatus of the present invention is able to examine 1000 detection 
signals in a time 2/3 times the time required by the conventional reticle 
inspecting apparatus. 
Referring to FIG. 32, a blocking circuit classifies detection signals 
provided by the detector according to magnitude into detection signals of 
three ranks, namely, those of a large rank (large foreign particle 
detection signals) corresponding to large foreign particles, those of a 
medium rank (medium foreign particle detection signals) corresponding to 
medium foreign particles and those of a small rank (small foreign particle 
detection signals) corresponding to small foreign particles, counts the 
respective numbers of large foreign particle detection signals, medium 
foreign particle detection signals and small foreign particle detection 
signals in each pixel block of 256 pixels (=16.times.16 pixels) and, only 
when the number of foreign particles in each pixel block is 1 or above, 
writes the respective numbers of the large, medium and small foreign 
particles included in each pixel block, the maximum detection signal among 
those provided by the pixels of each block and the coordinates of the each 
block in a storage device. 
A CPU sets a latch 4201 to a maximum number of foreign particles as an 
upper limit number of foreign particles to be detected. If the number of 
foreign particles exceeds the maximum number, the inspection is 
interrupted because the further inspection of a reticle having so many 
foreign particles is insignificant. A counter counts the number of 
detected foreign particles and a comparator 4211 compares the count of the 
counter 4221 with the maximum number to which the latch 4201 is set. If 
the count of the counter 4221 is greater than the maximum number, the 
inspection is interrupted. 
The CPU sets a latch 4202 to a high threshold for discriminating detection 
signals indicating large foreign particles from those indicating medium 
and small foreign particles. When the level of a detection signal is 
higher than the high threshold, it is decided that the detection signal 
indicates the detection of a large foreign particle. A comparator 4212 
compares the detection signal with the threshold and, if the detection 
signal is higher than the threshold, the count of a counter 4222 for 
counting the number of large foreign particles is incremented by one. 
The CPU sets a latch 4203 to a medium threshold for discriminating 
detection signals indicating medium foreign particles from those 
indicating small foreign particles. A comparator 4213 compares a detection 
signal with the medium threshold and, if the detection signal is higher 
than the medium threshold, it is decided that the detection signal 
indicates a medium foreign particle and the count of a counter 4223 is 
incremented by one. 
The CPU sets a latch 4204 to a low threshold for discriminating detection 
signals indicating small foreign particles from those indicating matters 
other than foreign particles. A comparator 4214 compares a detection 
signal with the low threshold and, if the detection signal is higher than 
the low threshold, it is decided that the detection signal indicates a 
small foreign particle and the count of a counter 4224 is incremented by 
one. 
In the foregoing foreign particle counting operation, the number of large 
particles is counted by all the counters, namely, the counter 4222 for 
counting large foreign particles, the counter 4223 for counting medium 
foreign particles and the counter 4224 for counting small foreign 
particles, and the number of medium foreign particles is counted by both 
the counter 4223 for counting medium foreign particles and the counter 
4224 for counting small foreign particles. Accordingly, the number of 
small foreign particles is determined by subtracting the number of medium 
foreign particles from the output of the counter 4224 for counting small 
foreign particles, and the number of medium particles is determined by 
subtracting the number of large foreign particles from the output of the 
counter 4223 for counting medium foreign particles. The respective numbers 
of large foreign particles, medium foreign particles and small foreign 
particles may be determined when displaying or providing the result of 
inspection. The detection signals may be compared by two comparators to 
make discrimination between detection signals indicating large, medium and 
small foreign particles. For example, only detection signals between a 
high threshold for large foreign particles and a medium threshold for 
medium foreign particles are selected as those indicating medium foreign 
particles, and only detection signals between the medium threshold and a 
low threshold for small foreign particles are selected as those indicating 
small foreign particles. 
Adders 4232, 4233 and 4234, and shift registers 4242, 4243 and 4244 blocks 
the array of one-dimensional detector, such as a CCD detector into 
two-dimensional blocks each of, for example, 16.times.16=256 pixels. The 
number of steps of the shift register is equal to (The number of pixels of 
the CCD array)/(The number of pixels on one side of the block). In this 
case, the number of pixels of the CCD array is 256 and the number of 
pixels on one side of the block is 16, and hence the number of steps of 
the shift register is 256/16=16. Although the number of steps of the shift 
register is equal to the number of pixels on one side of the block in this 
example, the coincidence of those numbers is an accident and the number of 
steps of the register and that of the pixels on one side of the block need 
not necessarily be equal to each other. However, if the value of (The 
number of pixels of the CCD array)/(The number of pixels on one side of 
the block) is not an integer, a blocking circuit having a complex 
configuration is necessary. Therefore, it is desirable to determine the 
number of pixels of the CCD array and the number of pixels on one side of 
the block so that the value of (The number of pixels of the CCD 
array)/(The number of pixels on one side of the block) is an integer. 
The contents of the counter 4222 for counting the number of large foreign 
particles are cleared (reset to zero) every time detection signals are 
provided by the pixels on one side of each block (sixteen pixels) are 
counted. A clear signal is obtained by dividing a clock which is provided 
for each of pixels arranged along the Y-axis of the detector by sixteen by 
means of a frequency divider (counter) 4261. In this case, a transfer 
clock of CCD array may be used as the clock for each of pixels arranged 
along the Y-axis. The count of the counter 4222 immediately before 
clearing, i.e., the count of detection signals for the sixteen pixels 
arranged along the Y-axis, is added to a value that appears at the output 
terminal of the 16-step shift register 4242 for large foreign particles by 
the adder 4232 and the output signal of the adder 4232 is applied to the 
input terminal of the 16-step shift register 4242 for large foreign 
particles. Then, the contents of the 16-step shift register 4242 is 
shifted by one step by the clear signal obtained by dividing the clock 
that is provided for each of the sixteen pixels arranged along the Y-axis. 
Therefore, the contents of the 16-step shift register 4242 is shifted by 
one step for every sixteen pixels arranged along the Y-axis. The contents 
of the 16-step shift register 4242 appears at the output terminal every 
time the contents of the same is shifted by sixteen steps. At this time, 
the CCD array is shifted a distance corresponding to one pixel along the 
X-axis and the number of large foreign particles detected by the sixteen 
pixels arranged along the Y-axis is added to the contents of the 16-step 
shift register 4242 by the adder 4232. The contents of the 16-step shift 
register 4242 is cleared by a signal obtained by dividing a pulse provided 
by an encoder provided every time the CCD array is shifted a distance 
corresponding to one pixel along the X-axis by sixteen by means of a 
frequency divider (counter) 4262; that is, the contents of the 16-step 
shift register 4242 is cleared every time the CCD array is shifted a 
distance corresponding to sixteen pixels arranged along the X-axis. 
Accordingly, the contents of the 16-step shift register 4242 for large 
foreign particles is the number of large particles detected by the 
16.times.16=256 pixels. When a comparator 4215 decides that the number of 
large foreign particles is not zero, signal representing the number of 
foreign particles and the coordinates of the block is given to a 
processing and memory means 4271, in which the number of foreign particles 
is the count of the counter 4224 equal to the sum of the respective 
numbers of detected large, medium and small foreign particles. Modes of 
operation of the blocking circuit for medium foreign particles and for 
small foreign particles are thee same as that for large foreign particles 
described above. 
A circuit for selecting the maximum detection signal from among those 
included in each block processes 16.times.16=256 pixels by a maximum 
detection signal selecting procedure, which is the same as the foreign 
particle counting procedure for counting the number of detected foreign 
particles except that the former procedure uses a latch 4205 for holding 
the maximum detection signal provided by one of the sixteen pixels 
arranged along the Y-axis instead of the latches 4201, 4202 and 4203 and 
the counters 4222, 4223 and 4224, and a comparator 4217 and a selector 
4251 instead of the adders 4232, 4233 and 4234, by using a clear signal 
for sixteen pixels arranged along the Y-axis, and a 16-step shift register 
4245. 
The storage device for storing foreign particle data stores (1) the number 
of detection signals indicating comparatively large foreign particles and 
exceeding the threshold for detection signals indicating comparatively 
large foreign particles, i.e., the number of comparatively large foreign 
particles in the block, (2) the number of detection signals indicating 
medium foreign particles and exceeding the threshold for detection signals 
indicating medium foreign particles, i.e., the number of medium foreign 
particles in the block, (3) the number of detection signals indicating 
comparatively small foreign particles and exceeding the threshold for for 
detection signals indicating comparatively small foreign particles, i.e., 
the number of comparatively small foreign particles in the block, (4) the 
maximum detection signal and (5) the coordinates of the block. 
These data for the block at the coordinates (5) stored in the storage 
device are read from the storage device sequentially to observe the 
foreign particles for confirmation. In some cases it is desirable to 
display all the data on a display. 
The detection sensitivity of an apparatus that carries out binarization in 
detecting foreign particles, such as the reticle inspecting apparatus of 
the present invention, is greatly dependent on the thresholds, 
particularly, on the threshold for the detection signals indicating 
comparatively small foreign particles. If the threshold is excessively 
large, the apparatus fails to find comparatively small foreign particles. 
Failure in detecting comparatively small particles will cause no problem 
if the apparatus is applied to the detection of foreign particles in a 
process to monitor the soundness of the process on the basis of the 
variation of the number of foreign particles. However, failure in 
detecting even comparatively small foreign particles on a photomask, such 
as a reticle, causes serious problems in the products manufactured by 
using the photomask in the exposure and printing processes. Therefore, 
when the reticle inspecting apparatus must not fail in detecting even a 
single comparatively small foreign particles on the reticle. To ensure 
successful detection of comparatively small foreign particles, it is 
desirable to reduce the threshold for the detection signals to the 
smallest possible extent. If the threshold is excessively small, in some 
cases, normal portions of a circuit pattern are detected in mistake for 
defects, such as foreign particles. The present invention enables the 
operator to fetch detection data from the storage device to examine the 
detection data to see whether or not the detection data indicates a 
foreign particle and, if the detection data is the result of erroneous 
detection, to delete the detection data. 
However, if the number of erroneously detected normal portions of the 
circuit pattern is very large (generally, the circuit patterns of most LSI 
circuits have millions of lines), the confirmation of the detection data 
requires a practically intolerable long time. Therefore, if the number of 
detection signals is excessively large (in such a case, most detection 
signals are those provided by the erroneous detection of normal portions 
of the circuit pattern), a new threshold larger than the threshold is set 
and the reticle inspection is repeated by using the new threshold, which, 
however requires additional time for repeating the reticle inspection. 
Therefore, it is desirable to employ a reticle inspecting method that 
repeats the inspection of a small inspection area requiring a short time 
to determine an appropriate threshold, and then starts the inspection of 
the entire area of the reticle after determining an appropriate threshold, 
a reticle inspecting method that decides that the threshold is 
inappropriate and interrupts the reticle inspecting operation when the 
number of detection signals increases beyond a predetermined value, a 
reticle inspecting method that interrupts the reticle inspecting operation 
when the number of detection signals increases beyond a predetermined 
value, increases the threshold by a predetermined value, and then restarts 
the reticle inspecting operation automatically, a reticle inspecting 
method that checks the number of detection signals provided in a fixed 
time interval, i.e., the rate of increase of the number of detection 
signals, and interrupts the reticle inspecting operation when the number 
of detection signals provided in the fixed time interval exceeds a 
predetermined value, or a reticle inspecting method that interrupts the 
reticle inspecting operation upon the increase of the number of detection 
signals provided in the fixed time interval beyond the predetermined 
value, increases the threshold by a predetermined value, and then restarts 
the reticle inspecting operation automatically. A latch 4201 shown in FIG. 
32 is an important component to realize one of those reticle inspecting 
method. 
It is also possible to store all the detection signals or the detection 
signals indicating defects, such as foreign particles, in the storage 
device, to set a threshold after the reticle has completely been inspected 
and to select the detection signals exceeding the threshold as those 
indicating foreign particles. This procedure, however, is unable to solve 
the problems imposed on the group processing circuit, such as "A large 
foreign particle is mistaken for a plurality of small foreign particles, 
and a long time is necessary for gaining access to and confirming the 
results of inspection." and "The grouping by software after the completion 
of inspection requires addition time.". 
Therefore, the present invention displays and gains access to the results 
of detection of defects, such as foreign particles, in blocks, and uses 
the detection data of defects, such as foreign particles, obtained by the 
operation of the block processing circuit shown in FIG. 32, stored in the 
storage device and including (1) the number of detection signals exceeding 
the threshold for selecting comparatively large foreign particles (the 
number of comparatively large foreign particles), (2) the number of 
detection signals exceeding the threshold for selecting comparatively 
large foreign particles and medium foreign particles (the number of 
comparatively large particles and medium foreign particles), (3) the 
number of detection signals exceeding the threshold for selecting 
comparatively small foreign particles, medium foreign particles and 
comparatively large foreign particles (the number of comparatively small 
foreign particles, medium foreign particles and comparatively large 
foreign particles), (4) the maximum detection signal and (5) the 
coordinates of the blocks. 
The number of blocks containing detection signals to be displayed and to be 
accessed can be reduced and can be carried out efficiently by gaining 
access to only the blocks containing detection signals exceeding the 
threshold for selecting medium foreign particles and comparatively large 
foreign particles or only the blocks containing detection signals 
exceeding the threshold for selecting comparatively large foreign 
particles. Although operation may be made for (1) to (3), the use of (4) 
is the quickest way of decision. 
If (4) is used for decision, the detection signals need not be compared 
with the three ranks, i.e., the large rank, the medium rank and the small 
rank, and detection signals may be compared with an optional set value. 
The optional set value is changed (increased) gradually and changed 
(decreased) until the number of the blocks decreases to a value proper for 
access and confirmation when displaying the blocks, and then access to the 
blocks is gained, the inspection is carried out again by using the 
decreased set value as a new threshold, or the foregoing operation may be 
controlled by an automatic sequence control method. 
FIG. 33 shows the functional relation between detection signals 4101 and 
4111 provided by the detectors, shading compensating circuits 113 and 123, 
four-pixel addition circuits 114 and 124, block processing circuits 58 and 
558, and results of defect detection. 
FIG. 48 shows a signal processing circuit formed by incorporating the 
shading compensating circuits 113 and 123, the four-pixel addition 
circuits 114 and 124 and the block processing circuits 58 and 558 into the 
signal processing circuit of FIG. 41. FIG. 49 shows a signal processing 
circuit formed by incorporating the shading compensating circuits 113 and 
123, the four-pixel addition circuits 114 and 124 and the block processing 
circuits 58 and 558 into the signal processing circuit of FIG. 42. FIG. 50 
shows a signal processing circuit formed by incorporating the shading 
compensating circuits 113 and 123, the four-pixel addition circuits 114 
and 124 and the block processing circuits 58 and 558 into the signal 
processing circuit of FIG. 43. FIG. 51 shows a signal processing circuit 
formed by incorporating the shading compensating circuits 113 and 123, the 
four-pixel addition circuits 114 and 124 and the block processing circuits 
58 and 558 into the signal processing circuit of FIG. 44. FIG. 52 shows a 
signal processing circuit formed by incorporating the shading compensating 
circuits 113 and 123, the four-pixel addition circuits 114 and 124 and the 
block processing circuits 58 and 558 into the signal processing circuit of 
FIG. 45, and FIG. 53 shows a signal processing circuit formed by 
incorporating the shading compensating circuits 113 and 123, the 
four-pixel addition circuits 114 and 124 and the block processing circuits 
58 and 558 to the signal processing circuit of FIG. 46. 
A reticle inspecting apparatus according to the present invention comprises 
a detection optical system provided with an optical system having NA of 
0.4 or above, disposed on the front side of the reticle and capable of 
illuminating the front surface of a reticle obliquely with a front 
illuminating light beam of approximately 780 nm in wavelength, 
illuminating the back surface of the reticle obliquely with a back 
illuminating light beam of approximately 488 nm in wavelength, of 
concentrating scattered light, of separating the scattered light according 
to wavelength, a reticle, the reticle is washed perfectly in a washing of 
intercepting diffracted light diffracted by a circuit pattern formed on 
the front surface of the substrate of the reticle with spatial filters 
disposed on the Fourier transform planes and of focusing the separated 
scattered light on detectors; a correcting circuit capable of correcting 
errors, which are attributable to irregular illumination, in detection 
signals provided by the detectors; addition circuits for adding the 
outputs of 2.times.2 pixels of the corresponding detectors; and a circuit 
for selecting the maximum sum of detection signals from among four sums of 
detection signals obtained by shifting the detector by a distance 
corresponding to one pixel in four directions. Thus, defects, such as 
minute foreign particles of sizes on the submicron order adhering to the 
substrate provided with the circuit pattern, such as a photomask, 
particularly, a reticle provided with a phase shift film for improving 
printing resolution can be easily and stable discriminated from the 
circuit pattern by a simple optical system. 
A photomask, such as a phase shift reticle, is fabricated by a photomask 
fabricating process shown in FIG. 65(A). A circuit pattern of a metal thin 
film and a phase shift film are formed on the front surface of a substrate 
in a circuit pattern forming step 651 to form a reticle, the reticle is 
washed perfectly in a washing step 652, and then the surface of the 
reticle is inspected for defects, such as foreign particles, by the 
reticle inspecting apparatus of the present invention in an inspecting 
step 653. If any detrimental foreign particles are found on the reticle, 
the reticle is washed and inspected again. This procedure is repeated 
until no foreign particle is found on the reticle. 
After thus perfectly cleaning the reticle, pellicles for preventing the 
contamination of the reticle with foreign particles are attached to the 
reticle in a pellicle attaching step 654, and then the surfaces of the 
pellicles are inspected for foreign particles by the reticle inspecting 
apparatus of the present invention in an inspecting step 655. If any 
foreign particles are found, the pellicles are removed, the reticle is 
washed again, and then the pellicle attaching step 654, the inspecting 
step 655, removal of the pellicles and washing are repeated until no 
foreign particle is found on the reticle. 
The perfect photomask, i.e., the perfect reticle, thus fabricated is 
delivered to an exposure process using a stepper. 
In a stepping projection process as shown in FIG. 65(B) using a stepper, a 
reticle is taken out from a stocker 656 storing reticles, and the reticle 
is inspected for foreign particles by the reticle inspecting apparatus of 
the present invention in an inspection step 657. If any foreign particle 
is found on the reticle, the reticle is returned to the reticle 
fabricating process, and the foreign particles are removed from the 
reticle by the reticle fabricating process. When no foreign particle is 
found on the reticle, the stepper carries out a dummy exposure step 658, 
the resist pattern formed on a wafer to be exposed to light transmitted by 
the reticle is inspected in a reticle error inspecting step 659 and, if 
the wafer is faultless, exposure and printing are carried out. FIG. 66 
also shows the stepping projection process. 
The reticle inspecting apparatus of the present invention is used in close 
systematic combination with the reticle fabricating process and the 
stepping projection process to supply perfect reticles perfectly free from 
defects, such as foreign particles, to semiconductor device fabricating 
processes for fabricating LSI circuits and the like.