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Description  |
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BACKGROUND OF THE INVENTION
The present invention relates to a photomask used in a semiconductor
fabricating apparatus, and more particularly to a method of and an
apparatus for detecting a defect on a photomask, in which a portion of a
light transmitting area is so formed as to shift the optical phase of
illumination light.
The resolving power of a projection aligner for projecting an image of a
mask pattern on an object can be improved by shifting the phase of light
passing through a photomask. In order to shift the phase of light passing
through the photomask, various methods have hitherto been used. For
example, a thin transparent film having a thickness corresponding to the
wavelength of illumination light which is used in the projection aligner
is formed at a desired position on a mask substrate, as described in
Japanese patent applications JP-A-58-173,744 and JP-A-57-62,052, or a
predetermined surface area of a mask substrate is etched to a
predetermined depth as described in a Japanese patent application
JP-A-62-189,468. Further, in the example shown in a Japanese patent
application JP-A-62-067,514, auxiliary patterns provided with a thin film
capable of reversing the phase of illumination light are added to a mask
pattern to improve the resolution of an image of the mask pattern.
In a conventional apparatus for detecting a defect on a photomask, a
photomask to be inspected is illuminated by ordinary illumination means,
and it is checked whether or not the light and darkness distribution on
that image of a mask pattern which is formed of light passing through the
photomask agrees with the light and darkness distribution on an image
which has a predetermined light-transmitting area and a predetermined
light-shielding area. For example, a method of comparing the light and
darkness distribution on the image formed of the transmitted light from
the photomask with the light and darkness distribution obtained from
design data which is recorded a magnetic tape is discussed on pages 138 to
144 of SPIE, Vol. 633, Optical Microlithography V (1986).
As can be seen from the above, the conventional apparatus for detecting a
defect on a photomask pays no attention to a defect in a phase-shifting
mask, and cannot detect a transparent or semitransparent defect such as a
transparent or semitransparent foreign substance attached to an ordinary
photomask.
The term "defect" used herein includes a defect in a thin film for
introducing the optical phase shifting, a defect in a chromium film for
forming a light shielding area, the remainder of etching, and others.
As mentioned above, the conventional apparatus for detecting a defect on a
photomask cannot detect a defect in a transparent film which is formed in
a light transmitting area to act as a phase shifter.
SUMMARY OF THE INVENTION
It is accordingly an object of the present invention to provide a method of
and an apparatus for detecting a defect in a transparent film formed in a
light transmitting area and a transparent or semitransparent foreign
substance attached to a photomask on the basis of a phase shift or
intensity change in either or both of transmitted and reflected light
beams from the photomask.
In order to attain the above object, a defect detecting apparatus according
to the present invention is provided with means for detecting either or
both of transmitted and reflected light beams from a photomask irradiated
with illumination light. In a case where a defect on a photomask is
detected only by the transmitted light, the photomask is illuminated with
coherent or partially coherent light emitted from a light source and
having a specified wavelength a to form the contour image of the defect by
the transmitted light from the photomask, thereby detecting the defect.
Further, in order to determine which of a light shielding area and a thin
film formed in a light transmitting area for shifting the phase of
illumination light has a defect, the illumination light from the light
source is separated by first beam splitting means into two light beams,
one of which is incident on the photomask The transmitted light from the
photomask is separated by second beam splitting means into two light
beams, one of which is detected by a first imaging device. The other light
beam from the first beam splitting means and the other light beam from the
second beam splitting means are caused to interfere, and the resultant
light thus obtained is detected by a second imaging device. Binary
information obtained from each of the first and second imaging devices for
indicating bright and dark states is used for determining where the defect
exists.
Further, in order to determine which of a light shielding area on a mask
substrate and a thin film for shifting the phase of illumination light has
a detect, the materials of the mask substrate and the thin film are
selected so that the mask substrate has a large transmittance for the
illumination light and the thin film has small transmittance. Then, the
transmitted light from the photomask has three intensity levels, and thus
three kinds of levels can be generated. It is determined from three kinds
of levels where the defect is present.
Further, according to the present invention, in order to control the
transmittance of a mask pattern for illumination light so that a defect on
the photomask can be detected, an absorbent used on the mask substrate,
the material of the thin film and the material of the mask substrate are
appropriately selected.
Further, in a case where the mask substrate and a phase shifter (that is, a
thin film) are different in refractive index from each other, or another
thin film different in refractive index from the phase shifter is
sandwiched between the mask substrate and the phase shifter, the defect
detecting apparatus according to the present invention is also provided
with means for detecting the reflected light from the photomask, and the
light and darkness distribution in the reflected light is compared with
that in the transmitted light to detect a defect in a light shielding film
or phase shifter.
Although various aspects of the present invention have been explained
above, a defect-detecting apparatus according to the present invention can
detect a defect in the phase shifter in the following manner. When
coherent or partially coherent light emitted from a light source passes
through a wavelength selection filter, coherent or partially coherent
light having a predetermined wavelength is obtained and used as
illumination light. A photomask to be inspected is locally provided with a
thin transparent film for shifting the phase of the illumination light.
Accordingly, when a defect is generated in the thin transparent film in
such a manner that a portion of the thin transparent film is removed for
some reason, a dark image is formed at the contour of the above portion
(namely, a film lacking area) on the basis of the interference between
light passing through the film lacking area and light passing through
outside of the film lacking area. In other words, the defect detecting
apparatus according to the present invention can illuminate a photomask
with light which is identical with illumination light used in a projection
aligner. When the light passing through the photomask is focused on an
imaging device, an image of a mask pattern is formed so that the contour
of the film lacking area is indicated by a dark line. By comparing the
pattern image with a non-defective image or an image reconstructed from
design data, the defect in the thin transparent film can be detected.
In a case where a mask substrate and a phase shifter (namely, a thin
transparent film for shifting the phase of illumination light) are
different in refractive index from each other, or another thin transparent
film different in refractive index from the phase shifter is interposed
between the mask substrate and the phase shifter, defect detection using
the reflected light beam from a photomask and defect detection using the
transmitted and reflected light beams from the photomask can be carried
out in addition to defect detection using the transmitted light beam from
the photomask.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram showing a first embodiment of an apparatus for
detecting a defect on a photomask in accordance with the present
invention.
FIG. 2A is a schematic diagram showing a non-defective mask pattern.
FIG. 2B is a schematic diagram showing the detected image of the mask
pattern of FIG. 2A.
FIG. 3A is a schematic diagram showing a defective mask pattern.
FIG. 3B is a schematic diagram showing the detected image of the mask
pattern of FIG. 3A.
FIG. 4 is a diagram showing a second embodiment of an apparatus for
detecting a defect on a photomask in accordance with the present invention
in which light passing through the photomask and a reference light
independent of the photomask are used for detecting the defect.
FIG. 5 is a diagram showing a third embodiment of an apparatus for
detecting a defect on a photomask in accordance with the present invention
in which embodiment light beams passing through adjacent points on the
photomask are used for detecting the defect.
FIG. 6 is a diagram showing a fourth embodiment of an apparatus for
detecting a defect on a photomask in accordance with the present invention
in which embodiment the quantity of light passing through the photomask is
measured to detect the defect.
FIG. 7 shows a photomask which can be inspected by the embodiment of FIG.
6.
FIG. 8 is a diagram showing a fifth embodiment of an apparatus for
detecting a defect on a photomask in accordance with the present invention
in which the light quantities of two wavelength components passing through
the photomask are measured to detect the defect.
FIG. 9A shows a mask pattern having no phase-shifting film, and detection
signals which are obtained by the embodiment of FIG. 8 in accordance with
the mask pattern.
FIG. 9B shows a mask pattern having a phase-shifting area, and detection
signals which are obtained by the embodiment of FIG. 8 in accordance with
the mask pattern.
FIG. 9C shows a mask pattern having a phase shifter in a light transmitting
area, and detection signals which are obtained by the embodiment of FIG. 8
in accordance with the mask pattern.
FIG. 10 is a diagram showing a sixth embodiment of an apparatus for
detecting a defect on a photomask in accordance with the present invention
in which the light quantities of transmitted and reflected light beams
from the photomask are measured to detect the defect.
FIG. 11 is a sectional view showing a photomask which can be inspected by
the embodiment of FIG. 10.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Embodiment I
Now, explanation will be made of a first embodiment of an apparatus for
detecting a defect on a photomask in accordance with the present invention
with reference to FIG. 1. Referring to FIG. 1, partially coherent light is
emitted from a light source 1 which is made up of a mercury lamp and an
aperture stop to be used as illumination light. The illumination light
from the light source 1 passes through a wavelength selection filter 2 and
a focusing lens 3, and is then incident on a photomask 5 which is fixed to
a predetermined position on a stage 4. The stage 4 is moved by drive means
6, and the position of the stage 4 is precisely measured by a laser
interferometer 7. The transmitted light from the photomask 5 passes
through another focusing lens 8 to form an image of a mask pattern on an
imaging device (that a, camera head) 9. The output signal of the imaging
device 9 is sent to a comparing/discriminating circuit 17 through a frame
memory 12 under control of a camera controller 10 to be compared with
design data for detection of a defect on the photomask 5. The output
signal of the imaging device 9 is also sent to a monitor 11 under control
of the camera controller 10. The comparing/discriminating circuit 17 is
made up of a magnetic tape (MT) 14, a buffer memory 15, a comparator 13
and a discriminating circuit 16. A defect on the photomask 5 can be
detected in the following manner. Design data recorded on the magnetic
tape 14 for forming a mask pattern is used for reconstructing a
predetermined image which is temporarily stored in the buffer memory 15.
Then, the contents of the frame memory 12 are compared with the contents
of the buffer memory 15 by the comparator 13, and the result of comparison
is sent to the discriminating circuit 16 to check whether a defect is
present or not.
Referring now to FIG. 2A, let us consider a case where a light shielding
area 22 of the photomask 5 includes two apertures 20-1 and 20-2 of the
same size, and only the aperture 20-2 is coated with a transparent,
non-defective film 21. Then, an image shown in FIG. 2B is stored in the
frame memory 12. Bright portions 23-1 and 23-2 shown in FIG. 2B correspond
to the apertures 20-1 and 20-2, respectively, and the portions 23-1 and
23-2 are equal in shape to each other. In a case where a part 24 of the
transparent film 21 is removed as shown in FIG. 3A, an image shown in FIG.
3B is sent to the comparing/discriminating circuit 17 through the frame
memory 12. As shown in FIG. 3B, a dark line 25 is formed along the contour
of the film lacking part or defect 24. This is because the wavelength of
the coherent or partially coherent illumination light is selected such
that the transparent film 21 shifts the phase of the illumination light by
a phase angle of 180.degree., and a dark image is formed at the contour of
the film lacking part 24 on the basis of interference between light
passing through, the film lacking part 24 and light passing through the
transparent film 21 outside of the film lacking part 24. The defect 24 in
the transparent film 21 can be detected by recognizing the dark line 25.
The image of the photomask 5 can be formed on the imaging device 9 at one
time. Alternatively, the illumination light is focused on the photomask 5
to form an image of a very fine area of the photomask 5 on the imaging
device 9, and the photomask 5 is scanned with the focused illumination
light to obtain the image of the whole of the photomask 5.
In the above explanation, the phase of the illumination light is shifted by
a thin transparent film having a predetermined thickness. The phase of the
illumination light can be shifted by other methods. For example, a mask
substrate may be selectively etched to a predetermined depth, or the mask
substrate is locally made high or low in refractive index. In either case
a defect in a region which serves as a phase shifter, can be detected by
the present embodiment in the form of the dark line 25 of FIG. 3B.
Embodiment II
FIG. 4 shows a second embodiment of an apparatus for detecting a defect on
a photomask in accordance with the present invention. According to the
present embodiment, in addition to the detection of the contour of a
defect in a thin transparent film serving as a phase shifter, it is
determined whether a defect bringing about a phase shift of 0.degree. is
present in a light transmitting area which produces a phase shift of
180.degree., or vice versa. Referring to FIG. 4, the illumination light
from the light source 1 passes through a collimator lens 3' and the
wavelength selection filter 2. The parallel rays thus obtained are
separated by a beam splitter 30-1 into two light beams, one of which is
directed to an imaging device 34 through a focusing lens 33-2 without
passing through the photomask 5 to be used as a reference light beam. The
other light beam is focused on the photomask 5 by a focusing lens 31. The
transmitted light from the photomask 5 passes through a collimator lens
32. The parallel rays thus obtained are separated by a beam splitter 30-2
into two light beams, one of which is incident on the imaging device 9
through a focusing lens 33-1. The other light beam from the beam splitter
30-2 and the reference light beam from the beam splitter 30-1 are caused
to interfere. The resultant light beam thus obtained is incident on the
imaging device 34. The position of the stage 4 mounted with the photomask
5 is always measured by the laser interferometer (not shown). That is, a
light focusing position on the photomask 5 is always measured.
According to the optical system shown in FIG. 4, when the illumination
light passing through the photomask 5 is not subjected to any phase shift,
bright light is incident on each of the imaging devices 9 and 34. Let us
consider a case where the photomask 5 is provided with a thin film for
shifting the phase of the illumination light by a phase angle of about
180.degree.. When each of the imaging devices 9 and 34 receives bright
light, it is determined that the thin film does not exist at the light
focusing position on the photomask 5. When the imaging device 9 receives
bright light and the imaging device 34 receives dark light, it is
determined that the thin film exists at the light focusing position on the
photomask 5. When, the imaging device 9 receives dark light, it is
determined that the light focusing position on the photomask lies in a
light shielding area. The output signals of the imaging devices 9 and 34
are applied to an arithmetic circuit 37 which produces three levels. These
levels are applied to the comparing/discriminating circuit 17 to be
compared with design data. Thus, a defect in the thin film serving as a
phase shifter can be detected. The arithmetic circuit 37 includes
comparators 41 and 42 for converting the output signals of the imaging
devices 9 and 34 into digital signals, and a level generator 43 for
generating three levels in accordance with the digital signals from the
comparators 41 and 42.
In a case where the phase of the light passing through the photomask
fluctuates widely because of the nonuniform thickness of a mask substrate,
the present embodiment may fail to detect a defect in the thin film
serving as a phase shifter. However, when the photomask 5 is scanned by
the focused illumination light, the time variation of transmitted light
quantity due to the nonuniform thickness of the mask substrate will be far
smaller than the time variation of transmitted light quantity due to the
presence or absence of the thin film. Accordingly, it is possible to
discriminate between a change in transmitted light quantity due to the
nonuniform thickness of the mask substrate and a change in transmitted
light quantity due to the thin film. In more detail, when a position on
the photomask 5 where the intensity of light due to interference between
the transmitted light and the reference light varies by more than a
predetermined amount is detected in a scanning period, such a position
represents the boundary between a light transmitting portion bringing
about no phase shift and a light transmitting portion bringing about a
phase shift of about 180.degree.. Accordingly, when the arithmetic circuit
37 has a function of detecting the intensity variation of light incident
on the imaging device 34 and a signal corresponding to the detected
intensity variation is applied to the comparing/discriminating circuit 17
to be compared with design data, the film lacking part 24 of FIG. 3A can
be detected.
Embodiment III
FIG. 5 shows a third embodiment of an apparatus for detecting a defect on a
photomask in accordance with the present invention. In the present
embodiment, as shown in FIG. 5, the illumination light is separated by the
beam splitter 30-1 into two light beams, and both of the light beams pass
through the photomask 5. Referring to FIG. 5, the above light beams are
focused on the photomask 5 so that the light focusing positions of the
light beams are spaced apart from each other by a distance of 2 to 5
.mu.m. After having passed through the photomask 5, one of the above light
beams is directed to the imaging device 34, and the other light beam is
separated by the beam splitter 30-2 into two light beams. One light beam
from the beam splitter 30-2 is received by the imaging device 9, and the
other light beam from the beam splitter 30-2 is directed to the imaging
device 34. Accordingly, light passing through one of the light focusing
positions on the photomask 5 and light passing through the other light
focusing position interfere, and the resultant light thus obtained is
received by the imaging device 34. Thus, when the contour of a defect in a
thin film (namely, phase shifter) lies between the light focusing
positions on the photomask 5, the imaging device 34 receives dark light.
As can be seen from the above explanation, a defective mask pattern
different from design data can be detected by processing the output
signals of the imaging devices 9 and 34 with the aid of the arithmetic
circuit 34 and comparing/discriminating circuit 17 which have been
explained in the
Embodiment II.
Further, in the present embodiment, the position of the stage 4 mounted
with the photomask 5 is always measured by the laser interferometer 7.
Hence, the position of the defect can be precisely detected. Additionally,
the light beams separated by the beam splitter 30-1 pass through light
focusing positions which lie in close proximity to each other, and hence
there is no fear of erroneously detecting a phase shift due to the
nonuniform thickness of a mask substrate.
Further, it is not always required to make the illumination light for
inspection equal in wavelength to the exposure light for projecting an
image of a mask pattern on a resist film or other surfaces. In a case
where the illumination light is different in wavelength from the exposure
light, the intensity of the light due to interference is not equal to
zero, but is far weaker than the intensity of transmitted light from the
photomask 5.
Embodiment IV
FIG. 6 shows a fourth embodiment of an apparatus for detecting a defect on
a photomask in accordance with the present invention. As shown in FIG. 6,
the present embodiment includes the light source 1, the focusing lens 3,
the stage 4 for holding the photomask 5 thereon, the focusing optical
system (that is, a receiving optical system) 8, a
transmitted-light-quantity measuring device 66, an A-D converter 67, a
memory 44, the level generator 43 for generating three levels, the drive
means 6 for driving the stage 4, and the laser interferometer 7. A
xenon-mercury lamp is used as the light source 1, and light emitted from
the light source 1 is incident on the filter 2. The filter 2 is selected
so that light having a wavelength of 254 nm passes through the filter 2.
The stage 4 has a structure that the photomask 5 can be fixed to a
predetermined position on the stage 4, and is moved by the drive means 6.
The position of the stage 4 is precisely measured by the laser
interferometer 7. The brightness at a desired position on the photomask 5
can be measured by the receiving optical system 8 and the
transmitted-light-quantity measuring device 66. A signal corresponding to
a received light quantity is sent from the device 66 to the A-D converter
67 to be converted into a digital signal. The digital signal thus obtained
is stored in the memory 44. The A-D converter 67 converts the input signal
thereto into the digital signal in synchronism with the output signal of
the laser interferometer 7 for measuring the position of the stage 4. The
level generator 43 compares information from the memory 44 with three
predetermined levels corresponding to a light shielding area, a light
transmitting area and a phase shifting area to generate three levels. The
levels thus obtained are applied to the comparing/discriminating circuit
17 to be compared with design data. Thus, a defect on the photomask 5 can
be detected.
The photomask used in the present embodiment has a structure shown in FIG.
7. That is, the photomask includes a quartz plate 71, a chromium film 72
for shielding light, and a thin film 73 serving as a phase shifter. The
thin film 73 is made of TSMR8800 (a trade name) manufactured by Tokyo Ohka
Co. Ltd. Alternatively, the thin film 73 may be formed of a resist film
which is made of a novolac resin other than TSMR8800. In the present
embodiment, illumination light having a wavelength of 254 nm is used for
detecting a defect on the photomask. In a case where the thin film 73 is
made of a novolac resin, light having a wavelength less than 340 nm can be
used for defect detection, since the transmittance of the thin film 73 is
decreased for light having a wavelength less than 340 nm.
The thin film 73 (that is, the phase shifter) which can be inspected by the
present embodiment may be formed of an SiN layer, an optical glass layer,
a silicon oxide layer containing at least one of titanium, lead, tin,
gold, indium, lanthanum, antimony, tantalum, yttrium, zirconium and
cerium, or an organic layer containing a benzene nucleus such as a
polystyrene layer. Additives such as titanium and lead act as an absorbent
for light having a specified wavelength. Incidentally, light having a
wavelength of 436 nm is used for projecting an image of the above
photomask on a resist layer. In a case where the thin film 73 is formed of
an organic layer containing a benzene nucleus, it is preferable to inspect
a defect in the thin film by light having a wavelength less than 280 nm,
since the transmittance of the organic layer is decreased for the above
light. It is very important to select the wavelength of the illumination
light for detecting a defect in the thin film 73 so that the transmittance
of the thin film is decreased for the selected wavelength. Accordingly,
the wavelength of the illumination light for detecting a defect in the
thin film may be made longer than the wavelength of exposure light for
projecting an image of the photomask on a resist layer.
The intensity distribution of transmitted light on the photomask 5 is
measured by the present embodiment, and the photomask 5 is divided into a
transparent area, a semi-transparent area and a light shielding area in
accordance with the intensity of the transmitted light. These areas are
compared by the comparing/discriminating circuit 17 with design data to
detect a pin hole and a film lacking portion. That is, it is checked
whether or not an area where a chromium film is to be deposited is
coincident with the light shielding area, whether or not an area where a
phase shifter is to be deposited is coincident with the semitransparent
area, and whether or not the exposed surface area of the quartz substrate
71 (the light transmitting area without the phase shifter) is coincident
with the transparent area.
The phase shifting area is transparent for the exposure light used for
projecting an image of a mask pattern on a resist layer. Accordingly, the
conventional defect detection method cannot discriminate between the phase
shifting area and the exposed surface area of a transparent substrate.
According to the present embodiment, the illumination light for detecting
a defect in the phase shifter is made different in wavelength from the
exposure light for projecting an image of the mask pattern on the resist
film, and the material of the phase shifter is selected so that the phase
shifter is semi-transparent for the defect detecting light. Thus, a defect
in the phase shifter can be detected by the present embodiment. It is
needless to say that the present embodiment can also detect a defect in
each of the light shielding area and the exposed surface area of the mask
substrate.
The present embodiment utilizes a fact that the transmittance of a phase
shifter is decreased for light lying in a wavelength range Alternatively,
the phase shifter may be formed of a polarizer. In this case,
linearly-polarized light inclined at 45.degree. with the plane of
polarization of the polarizer is used as illumination light. Then, the
chromium film intercepts the illumination light, the quantity of light
passing through the polarizer is one half the quantity of the illumination
light incident thereon, and the illumination light passing through the
exposed surface area of the transparent substrate is scarcely absorbed by
transparent substrate.
Embodiment V
FIG. 8 shows a fifth embodiment of an apparatus for detecting a defect on a
photomask in accordance with the present invention. As shown in FIG. 8,
the present embodiment includes the light source 1, a band pass filter 82,
an illuminating optical system 83, the stage 4 for holding the photomask 5
thereon, a receiving optical system 85, a beam splitter 86, band pass
filters 87a and 87b, photo-sensors 88a and 88b, the arithmetic circuit 37,
the comparing/discriminating circuit 17, and the drive means 6 for moving
the stage 4. A mercury-xenon lamp is used as the light source 1, and the
filter 82 transmits only a wavelength component of 254 nm and a wavelength
component of 436 nm. These wavelength components pass through the
illuminating optical system 83, and are then incident on the photomask 5
which is fixed to the stage 4. As in the fourth embodiment, the photomask
5 has the structure shown in FIG. 7. The thickness of the phase shifter 73
shown in FIG. 7 is selected so that the phase shifter 73 shifts the phase
of the wavelength component of 436 nm by a phase angle of .pi. or
180.degree..
Light passing through the photomask 5 is incident on the photosensors 88a
and 88b through the receiving optical system 85, the beam splitter 86, and
the filters 87a and 87b. The filter 87a can transmit only the wavelength
component of 436 nm, and the filter 87b can transmit only the wavelength
component of 254 nm. The rece | | |
