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Claims  |
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What is claimed is:
1. Apparatus for inspecting masks to detect defects comprising:
a mask station for mounting a mask having features to be inspected;
means for projecting radiation through said mask;
means for detecting the orientations of said features;
means for suppressing the optical signal from the features corresponding to
the determined orientations of the mask features; and
means for detecting defects in said mask and outputting indicia thereof.
2. Apparatus for inspecting masks to detect defects according to claim 1
wherein said means for detecting the orientations of the mask features
comprises a plurality of strip detectors disposed in a cylinder-like
configuration so that the axis of the beam is along the axis of the
cylinder, whereby orientation is determined by which detectors are
energized.
3. Apparatus for inspecting masks to detect defects according to claim 1,
wherein the projecting radiation is in the form of a beam having a
diameter of less than about 1 mil of an inch.
4. Apparatus for inspecting masks to detect defects according to claim 1
wherein said means for detecting the orientation of the mask features
comprises a substantially circular configuration of individual detectors
disposed around the optical axis in a pie-like array with the portion near
the center removed.
5. Apparatus for inspecting masks to detect defects according to claim 1
wherein said means for detecting the orientation of the mask features
comprises a substantially rectangular array of detectors.
6. Apparatus for inspecting masks to detect defects according to claim 1
wherein said means for projecting radiation comprises a first source for
projecting an orientation beam to said mask station and a second separate
source for projecting a defect detection beam to said mask station.
7. Apparatus for inspecting masks to detect defects according to claim 6
further comprising means for discriminating the signals from the
orientation beam and from the defect detection beam.
8. Apparatus for inspecting masks to detect defects according to claim 7
wherein said means for discriminating signals from the orientation beam
and from the defect detection beam comprises a beam splitter.
9. Apparatus for inspecting masks to detect defects according to claim 6
wherein said orientation beam and said defect detection beam have
different optical wavelengths.
10. Apparatus for inspecting masks to detect defects according to claim 6
wherein said orientation beam and said defect detection beam have
different modulation frequencies.
11. Apparatus for inspecting masks to detect defects according to claim 6,
wherein the diameters of said beams are less than about 1 mil of an inch
respectively, and wherein said means for detecting the orientations and
said means for detecting defects comprises an array of detectors mounted
adjacent said mask, and means for gating said array of detectors according
to the orientation of the regularly shaped mask features, and means for
detecting defects in said mask using said gated array of detectors.
12. Apparatus for inspecting masks to detect defects comprising:
a mask station for mounting a mask having features to be inspected;
source means for projecting radiation through said mask;
means for forming an orientation beam;
means for forming a defect detection beam;
a controller;
means for detecting the orientation of the mask features and outputting a
corresponding signal to said controller using said orientation beam; and
means for detecting defects in said mask and outputting indicia thereof
using said defect detection beam, said controller controlling said means
for detecting defects corresponding to the orientation of the mask
features.
13. Apparatus for inspecting masks to detect defects according to claim 12
wherein said means for detecting the orientation of the mask features
comprises a plurality of strip detectors disposed in a cylinder-like
configuration so that the axis of the beam is along the axis of the
cylinder, whereby orientation is determined by which detectors are
energized.
14. Apparatus for inspecting masks to detect defects according to claim 12,
wherein the diameters of said beams are less than about 1 mil of an inch,
respectively.
15. Apparatus for inspecting masks to detect defects according to claim 12
wherein said means for detecting the orientation of the mask features
comprises a substantially circular configuration of individual detectors
disposed around the optical axis in a pie-like array with the portion near
the center removed.
16. Apparatus for inspecting masks to detect defects according to claim 12
wherein said means for detecting the orientation of the mask features
comprises a substantially rectangular array of detectors.
17. Apparatus for inspecting masks to detect defects according to claim 12
wherein said source means comprises a first source for projecting the
orientation beam to said mask station and a second separate source for
projecting the defect detection beam to said mask station.
18. Apparatus for inspecting masks to detect defects according to claim 12
further comprising means for discriminating the signals from the
orientation beam and from the defect detection beam.
19. Apparatus for inspecting masks to detect defects according to claim 18
wherein said means for discriminating signals from the orientation beam
and from the defect detection beam comprises a beam splitter.
20. Apparatus for inspecting masks to detect defects according to claim 12
wherein said orientation beam and said defect detection beam have
different optical wavelengths.
21. Apparatus for inspecting masks to detect defects according to claim 12
wherein said orientation beam and said defect detection beam have
different modulation frequencies.
22. Apparatus for inspecting masks to detect defects according to claim 12,
wherein the diameters of said beams are less than about 1 mil of an inch
respectively, and wherein said means for detecting the orientation and
said means for detecting defects comprise an array of detectors mounted
adjacent said mask, and means for gating said array of detectors according
to the orientation of the regularly shaped mask features, and means for
detecting defects in said mask using said gated array of detectors.
23. Apparatus for inspecting masks to detect defects comrising:
a mask station for mounting a mask having features to be inspected;
source means for projecting radiation through said mask;
means for forming an orientation beam;
means for forming a defect detection beam;
a controller;
a transform lens, mounted in the path of said radiation, for focusing the
radiation transmitted through said mask in the transform plane of said
mask;
means for detecting the orientation of the mask features using said
orientation beam and outputting a corresponding signal to said controller;
and
means for detecting defects in said mask using said defect detection beam
and outputting indicia thereof, said controller controlling said means for
detecting defects corresponding to the orientation of the mask features.
24. Apparatus for inspecting masks to detect defects acording to claim 23
wherein said means for detecting the orientation of the mask features
comprises a plurality of strip detectors disposed in a cylinder-like
configuration so that the axis of the beam is along the axis of the
cylinder, whereby orientation is determined by which detectors are
energized.
25. Apparatus for inspecting masks to detect defects according to claim 23,
wherein the diameters of said beams are less than about 1 mil of an inch,
respectively.
26. Apparatus for inspecting masks to detect defects according to claim 23
wherein said means for detecting the orientation of the mask features
comprises a substantially circular configuration of individual detectors
disposed around the optical axis in a pie-like array with the portion near
the center removed.
27. Apparatus for inspecting masks to detect defects according to claim 23
wherein said means for detecting the orientation of the mask features
comprises a substantially rectangular array of detectors.
28. Apparatus for inspecting masks to detect defects according to claim 23
wherein said source means comprises a first source for projecting the
orientation beam to said mask station and a second separate source for
projecting the defect detection beam to said mask station.
29. Apparatus for inspecting masks to detect defects according to claim 23
further comprising means for discriminating the signals from the
orientation beam and from the defect detection beam.
30. Apparatus for inspecting masks to detect defects according to claim 29
wherein said means for discriminating signals from the orientation beam
and from the defect detection beam comprises a beam-splitter.
31. Apparatus for inspecting masks to detect defects according to claim 23
wherein said orientation beam and said defect detection beam have
different optical wavelengths.
32. Apparatus for inspecting masks to detect defects according to claim 23
wherein said orientation beam and said defect detection beam have
different modulation frequencies.
33. Apparatus for inspecting masks to detect defects according to claim 23,
wherein the diameters of said beams are less than about 1 mil of an inch
respectively, and wherein said means for detecting the orientation and
said means for detecting defects comprises an array of detectors mounted
adjacent said mask, means for gating said array of detectors according to
the orientation of the regularly shaped mask features, and means for
detecting defects in said mask using said gated array of detectors.
34. Apparatus for inspecting masks to detect defects according to claim 23
further comprising a second distinguishable defect detection beam for
detecting equivalent mask areas with respect to the first defect detection
beam, and wherein said means for detecting defects in said mask comprises
means for detecting defects in a first area of said mask using said first
defect detection beam and means for detecting defects in a second
equivalent area of said mask using said second defect detection beam.
35. Apparatus for inspecting masks to detect defects comprising:
an optical system having a Fourier transform plane;
means for mounting a mask having regularly shaped mask features in said
optical system;
means mounted substantially in said transform plane for suppressing the
regularly shaped mask features and passing light tracings of defects;
means for detecting the orientation of the regularly shaped mask features;
means for adjusting the means for suppressing the regularly shaped masked
features based on the determined orientation of the mask features; and
means for detecting said passed light to produce an output indicative of
the presence of defects.
36. Apparatus for inspecting masks to detect defects comprising:
an optical system having a Fourier transform plane;
means for mounting a mask having regularly shaped mask features in said
optical system;
an approximate form factor intensity spatial filter mounted substantially
in said transform plane for suppressing the regularly shaped masked
features and passing light tracings of defects;
means for detecting the orientation of the mask features;
means for adjusting said spatial filter based on the determined orientation
of the mask features;
means for detecting said passed light to produce an output; and
means for utilizing said output to generate indicia of defects in said
mask.
37. Method of detecting defects in a photomask containing circuit elements,
comprising the steps of:
mounting a mask having features to be inspected in a mask station;
projecting radiation through said mask;
detecting the orientations of said features;
suppressing the optical signal from the features corresponding to the
determined orientation of the mask features; and
detecting defects in said mask and outputting indicia thereof.
38. Method of detecting defects in a photomask containing circuit elements,
comprising the steps of:
mounting a mask having features to be inspected in a mask station;
projecting radiation through said mask;
forming an orientation beam;
forming a defect detection beam;
detecting the orientation of the mask features and outputting a
corresponding signal to a controller using said orientation beam; and
detecting defects in said mask and outputting indicia thereof using said
defect detection beam which is controlled by said controller corresponding
to the orientation of the mask features.
39. Method of detecting defects in a photomask containing circuit elements,
comprising the steps of:
mounting a mask having features to be inspected in a mask station;
projecting radiation through said mask;
forming an orientation beam;
forming a defect detection beam;
mounting a transform lens in the path of said radiation for focusing the
radiation transmitted through said mask in the transform plane of said
mask;
detecting the orientation of the mask features using said orientation beam
and outputting a corresponding signal to a controller; and
detecting defects in said mask using said defect detection beam controlled
by said controller corresponding to the orientation of the mask features
and outputting indicia thereof.
40. Method of detecting defects in a photomask containing circuit elements,
comprising the steps of:
mounting a mask having regularly shaped mask features in a optical system
having a Fourier transform plane;
mounting an approximate form factor intensity spatial filter substantially
in said transform plane for suppressing the regularly shaped masked
features and passing light tracings of defects;
detecting the orientation of the mask features;
adjusting said spatial filter based on the determined orientation of the
mask features;
detecting said passed light to produce an output; and
utilizing said output to generate indicia of defects in said mask. |
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Claims |
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Description |
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FIELD OF INVENTION
This invention relates to photolithography for integrated circuits and,
more particularly, to method and apparatus for inspecting photomasks to
detect defects.
BACKGROUND OF INVENTION
Reference is made to my U.S. Pat. No. 3,658,420 issued Apr. 25, 1972, which
describes an optical spatial filtering technique for detecting hole-type
defects and excess spot defects in photomasks used in making
micro-circuits. According to the patent, an approximate form factor
intensity filter provides suppression of the regularly shaped mask
features. Thus, for masks with features whose boundaries are along only
the X-Y direction, the filter is a cross placed in the transform plane.
With the rectangular features suppressed, primarily non-rectangular defect
data passes so that spots as small as 0.1 mil are detected.
The present invention is directed to improvements over the methods of my
earlier patent, as will become apparent as the description proceeds.
SUMMARY OF THE INVENTION
It is one object of the present invention to provide means for detecting
the orientation of the mask features of the optical design. It is another
object of the invention to provide means for altering the spatial
filtering responsive to the detected orientation of the features so that
the optical design from the mask features can be suppressed by suitable
spatial filtering.
In order to accomplish the desired results, the invention provides a new
and improved method and apparatus for detecting defects in photomasks
containing features with different arbitrary orientations. Said apparatus
includes in one form thereof detector means for detecting the orientations
of the features so that the optical signals from the features may be
suppressed by suitable means. One suitable means is spatial filtering. The
spatial filtering is adjusted electronically based on the determined
orientation of the mask features. In one form of the invention, the
spatial filtering is effected by accepting photo detected signals from
only a select number of photodetectors in a photodetector array, the
selection being determined from the orientation of the mask features. In
one form of the invention a second distinguishable defect detection beam
is employed for detecting equivalent mask areas with respect to the first
defect detection beam and the means for detecting defects in the mask
comprises means for detecting defects in a first area of the mask using
the first defect detection beam and means for detecting defects in a
second equivalent area of said mask using said second defect detection
beam.
There has thus been outlined rather broadly the more important features of
the invention in order that the detailed decription thereof that follows
may be better understood, and in order that the present contribution to
the art will appreciate that the conception upon which the disclosure is
based may readily be utilized as a basis for the designing of other method
and apparatus for carrying out the several purposes of the invention. It
is important, therefore, that the Claims be regarded as including such
equivalent method and apparatus as do not depart from the spirit and scope
of the invention.
Several embodiments of the invention have been chosen for purpose of
illustration and description, and are shown in the accompanying drawings,
forming a part of the specification.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic block diagram of an apparatus for inspecting masks to
detect defects thereon constructed according to the concepts of the
present invention;
FIG. 2 is a schematic block diagram of apparatus for inspecting masks to
detect defects thereon similar to FIG. 1, but showing another embodiment
of the invention;
FIG. 3 is a schematic block diagram of another embodiment of the invention;
FIG. 4 is a schematic block diagram similar to FIGS. 1-3, but showing still
another embodiment of the invention;
FIG. 5 is a schematic plan view of a detector array according to one form
of the invention;
FIG. 6 is a schematic plan view of another form of the detector array
according to the invention; and
FIG. 7 is a schematic perspective view of still another form of detector
array according to the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In the embodiment of the invention illustrated in FIG. 1, apparatus for
inspecting photomasks to detect defects is illustrated, which comprises a
mask station 10 for mounting a mask, which is to be tested for defects.
Movement of the mask into and out of position is controlled by controller
means 12. Controller means 12 serves to control and coordinate the
functions of the elements in the apparatus, as will be described more
fully hereinafter. Source means 14 are provided for projecting radiation
through the mask. This can be coherent or incoherent radiation and may or
may not be in the visible range.
From the mask station the beam or beams pass to a Fourier transform lens
16, which focuses the radiation transmitted through the mask in the
transform plane of the lens. In this embodiment of the invention detector
means 18, under control of the controller 12, serves the multiple purposes
of functioning as a spatial filter, as a demodulator or beam separator and
as a detector. When functioning as a filter, the detector suppresses the
regularly shaped mask features and detects mask defects from the
remainder. Thus, this detector includes means 20 for detecting the
orientation of the mask features using an orientation beam and outputting
a corresponding signal to the controller 12, which adjusts the detector in
a corresponding manner. This detector also includes means 22 for detecting
defects in the mask using a defect detection beam and outputting indicia,
as at 24.
In one form of the invention, the mask orientation detector 20 could be in
a pie-shaped configuration, as shown in FIG. 5. Thus, an array of
detectors 26 appears as slices in a pie with a portion 28 near the center
removed. The use of the detector array with the detectors in the form of
pie slices eliminates the need for computer data acquisition and
processing and permits hard-wired operation with relatively simple
hardware.
In another embodiment of the invention, the detector means is in the form
of a rectangular array of detectors, as indicated at 30 in FIG. 6. When
the beam from the mask strikes the detectors, information is signaled to
the controller indicating the orientation of the mask features. The
regularly shaped mask features could, for example, be in the form of a
cross or a line, as indicated at 32. As a result the array of detectors is
gated according to the orientation of the regularly shaped mask features.
That is, the detectors 34 within the cross-shaped area are deactivated
while the remaining detectors are activated, so that when the radiation
from the mask is directed to the array of detectors, the activated
detectors output indicia of the presence of defects in the mask.
FIG. 7 illustrates another desirable form of detector means for detecting
the orientation of the mask features. The detector of FIG. 7 includes a
plurality of strip detectors 36 disposed in a cylinder-like configuration
so that the axis of the beam, indicated by an arrow 38, is coaxial with
the axis of the cylinder, whereby orientation is determined by which
detectors are energized.
FIG. 2 shows another embodiment of the invention which includes a mask
station 40 for mounting a mask, which is to be tested for defects. The
mask can be moved into and out of position as controlled by controller
means 42.
As pointed out in connection with the embodiment of FIG. 1, the controller
serves to control and coordinate the function of the elements in the
apparatus. Source means are provided for projecting radiation through the
mask. As pointed out above, this can be coherent or incoherent radiation
and may or may not be in the visible range. There is provided an
orientation beam 44, a first defect detection beam 46 and a second defect
detection beam 48, FIG. 2. These beams are passed to the mask station 40,
through modulators 50, 52 and 54, respectively. Any suitable type of
modulator may be employed such as, frequency modulation means, amplitude
modulation means, wavelength modulation means or modulation using
alternating beams, for example. These modulators are controlled by the
controller 42.
Still referring to the embodiment of FIG. 2, from the mask station 40 the
beam or beams pass through a Fourier transform lens 56, which focuses the
radiation transmitted through the mask in the transform plane of the lens.
In another form the beam or beams from the transform lens pass through
filter means which could include, a first filter means 58, second filter
means 60 and third filter means 62, for example. The first filter means
serves to form an orientation beam, which is directed to an orientation
detector 64. The second filter means serves to form a first defect
detection beam, which is directed to a first defect detector 66. The third
filter means serves to form a second defect detection beam, which is
directed to a second defect detector 68. Any suitable form of detectors
may be employed, such as, for example, those described in connection with
the embodiment of FIG. 1.
In operation, the signal detected by the orientation detector 64 is sent to
the controller 42, which adjusts the filter means or the defect detectors
in accordance with the orientation of the mask features in response so
that the first defect detector 66 outputs defect indicia as at 70 and the
second defect detector 68 if employed outputs defect indicia as at 72.
In operation, in some installations the second defect detection beam 48,
modulator 54, third filter means 62 and second defect detector 68 may not
be employed. However, most masks contain a plurality of identical,
repetitive, equivalent, circuit patterns. When these elements are
employed, inspective of two or more equivalent circuit patterns can be
done using one optical system. Thus, signals from two areas or more, can
be distinguished by modulation of the two illuminating beams such as at
different temporal frequencies, for example. In this embodiment, the
defect detection means 68 followed by the filter means 62 futher separates
the second defect dectection beam from the first defect detection beam and
the orientation beam, so that the detector 66 detects defects in the first
area of the mask using the first defect detection beam and the detector 68
detects defects in the second area of the mask using the second defect
detection beam and outputting indicia thereof at 70 and 72, respectively.
Referring next to the form of the invention illustrated in FIG. 3, this
form is similar to the forms of the invention illustrated in FIGS. 1 and
2, except that the Fourier transform lens has been excluded. The apparatus
detects the beam passing through the mask directly with a detector. A
source 76, modulator 78, mask station 80, controller 82 may be of the same
configuration and function as the corresponding elements described in
connection with the embodiments of FIGS. 1 and 2. However, in this case it
is essential that the beams be very small at the mask such as, for
example, a diameter of less than about 1 mil of an inch each. Any suitable
type of detectors may be employed such as, for example, the ones described
in connection with FIGS. 5, 6 and 7.
Still another embodiment of the invention is illustrated in FIG. 4. There
is provided an orientation beam 84, a first defect detection beam 86 and a
second defect detection beam 88. The beams pass through modulators 92, 94
and 96, respectively. Any suitable type of modulator may be employed, such
as, frequency modulation means, amplitude modulation means, wave length
modulation means or modulation using alternating beams, for example. These
modulators as well as the beams 84, 86 and 88 are controlled by a
controller 98.
From the mask station 90, the beam or beams pass to a Fourier transform
lens 100, which focuses the radiation transmitted through the mask in the
transform plane of the lens. In the embodiment illustrated in FIG. 4, the
spatial filter means 102 are mounted substantially in said transform plane
for suppressing the regularly shaped mask features and passing light
tracings of defects when it has been oriented to the features of the mask.
The orientation of the spatial filter 102 is controlled by controller 98.
Subsequent to the spatial filter one or more of several elements may be
employed, which could be employed in several possible sequences. That is,
in some installations, depending on the beams passing through the mask,
only detector means 104 need be employed. In this case the detector means
could be of any suitable type such as one which serves the multiple
purposes of functioning as a demodulator and as a detector. In other forms
of the invention a demodulator 106 is mounted before the detector 104, and
beamsplitter 108 is mounted before detector 104. Any suitable type of
demodulator may be employed such as, for example, one which separates the
orientation beam, the first detection beam and the second detection beam
one from the others. The type and characteristics of the demodulator are,
of course, dependent on the type and characteristics of the beams or
modulators supplied. The beam splitter 108 serves to split the beam into
an orientation beam, a first detection beam and a second detection beam.
The beam splitter could be dichroic whereby the beams could be split
according to wavelength. In any case, signals corresponding to the
orientation of the features of the mask are sent to the controller 98,
which then adjusts the spatial filter means 102 to pass only the defects
so that the detector means 104 outputs a signal as at 110 indicating the
presence or absence of defects in the mask. Similar to the embodiment of
FIG. 2, this embodiment could be either in a form employing a second
detection beam and detector or in a form excluding these elements. In some
installations it may be desirable to employ a large number of individual
defect detection beams with corresponding detectors.
It will thus be appreciated that the present invention does indeed provide
new and improved method and apparatus for inspecting masks to detect
defects, which reduces the computer processing required from a detector
array, and which is simpler and more accurate and faster than such
apparatus employed heretofore.
Although specific embodiments have been illustrated and described, it will
be obvious to those skilled in the art that various modifications may be
made without departing from the spirit and scope of the invention, which
is to be limited solely by the appended claims.
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Description |
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