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Claims  |
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What is claimed is:
1. A method of detecting in a light pattern the presence of defects in a
specimen subject to which the light pattern corresponds, comprising:
providing a light detector to detect light from the light pattern, the
light detector comprising light detecting elements arranged in a first
array of rows and columns and defining in the light pattern plural
adjacent stripe regions each of which includes plural pixel elements
arranged in a second array of rows and columns, and each light detecting
element being operable to provide a measured energy value corresponding to
the amount of light present in any one of the pixel elements;
successively aligning each one of the light detecting elements in a column
of the first array with a pixel element so that each one of the light
detecting elements in the column acquires an energy value corresponding to
the pixel element, thereby to acquire for the pixel element a number of
energy values that equals the number of light detecting elements in the
column;
accumulating the number of energy values to provide a total energy value
corresponding to the amount of light present in the pixel element and
proportional to the sum of the number of energy values; and
determining from the total energy value whether the amount of light present
in the pixel element represents a defect in the specimen subject.
2. The method of claim 1 in which the light pattern is stationary and the
successive alignment of the light detecting elements in the column is
accomplished by continuously moving the first array of light detecting
elements along the light pattern stripe region in which the pixel element
is located.
3. The method of claim 1 in which each one of the light detecting elements
in the column comprises a part of a charge-coupled device and produces a
quantity of charge which represents an energy value, and the number of
energy values are accumulated to provide the total energy value by moving
the light detecting elements in the column in a scan direction serially
past the pixel element and shifting in a direction opposite the scan
direction along the column the quantity of charge measured by the light
detecting element in present alignment with the pixel element to the next
adjacent light detecting element in the column.
4. Apparatus for detecting in a light pattern the presence of defects in a
specimen subject to which the light pattern corresponds, comprising:
light detecting means for detecting light from the light pattern, the light
detecting means comprising light detecting elements arranged in a first
array of rows and columns and defining in the light pattern plural
adjacent stripe regions each of which includes plural pixel elements
arranged in a second array of rows and columns, and each light detecting
element being operable to provide a measured energy value corresponding to
the amount of light present in any one of the pixel elements;
scanning means to scan the light detecting means along a stripe region so
that in succession each light detecting element in one column of the first
array traverses and acquires an energy value corresponding to the amount
of light present in a pixel element in one column of the second array;
accumulating means to accumulate a total energy value proprotional to the
sum of the energy values acquired for the pixel element by all of the
light detecting elements in the one column of the first array; and
means to determine from the total energy value whether the amount of light
in the pixel element represents a defect in the specimen subject.
5. The apparatus of claim 4 in which the light detecting means comprises a
charge-coupled device.
6. The apparatus of claim 4 in which the light pattern remains stationary
and the scanning means further comprises positioning means to position the
light detecting means across each one of the stripe regions to scan them
in a serial manner.
7. The apparatus of claim 6 in which the positioning means moves the light
detecting means continuously across each stripe region.
8. The apparatus of claim 4 in which the first array has a first row and N
total number of rows and which further comprises position-detecting means
for detecting the position of the first array relative to the stripe
region, the position-detecting means cooperating with the accumulating
means so that each one of the lihgt detecting elements in the first row of
the one column never accumulates more than one energy value for any one of
the pixel elements of the second array with which it becomes aligned, and
each one of the light detecting elements in the Nth row of the one column
has accumulated N number of energy values for any one of the pixel
elements with which it becomes aligned.
9. The apparatus of claim 4 in which the light pattern is derived from
reconstruction of a hologram that corresponds to the specimen pattern.
10. The appartus of claim 4 in which the specimen pattern is that of a
photomask.
11. Apparatus for detecting a light pattern of relatively low intensity,
comprising:
light detecting means including a first array of light detecting elements
arranged in rows and columns to detect light from the light pattern, the
light detecting means defining in the light pattern plural adjacent stripe
regions each of which includes plural pixel elements arranged in a second
array of rows and columns, and each light detecting element comprising a
part of a charge-coupled device and being operable to produce a quantity
of charge corresponding to the amuont of light present in any one of the
pixel elements;
scanning means to scan the light detecting means along a stripe region so
that each light detecting element in one column of the first array
traverses each pixel element in one column of the second array; and
accumulating means to accumulate for each one of the pixel elements in the
one column of the second array at total quantity of charge proportional to
the sum of the quantities of charge acquired by all of the light detecting
elements in the one column of the first array, the total quantity of
charge being accumulated by moving the light detecting elements in the
column in a scan direction serially past the pixel element and shifting in
a direction opposite the scan direction along the column the quantity of
charge measured by the light detecting element in present alignment with
the pixel element to the next adjacent light detecting element in the
column.
12. The apparatus of claim 11 in which the light pattern remains stationary
and the scanning means further comprises positioning means to position the
light detecting means across each one of the stripe regions to scan them
in a serial manner.
13. The apparatus of claim 12 in which the scanning means moves the light
detecting means continuously across each stripe region.
14. A method of detecting in a light pattern the presence of defects in a
specimen subject to which the light pattern corresponds, comprising:
providing a light detector to detect light from the light pattern, the
light detector comprising light detecting elements arranged in a first
array of rows and columns and defining in the light pattern plural
adjacent stripe regions each of which includes plural pixel elements
arranged in a second array of rows and columns, and each one of the light
detecting elements in a column comprising a part of a charge-coupled
device and being operable to produce a quantity of charge which represents
an energy value corresponding to the amount of light present in any one of
the pixel elements;
successively aligning each one of the light detecting elements in a column
of the first array with a pixel element to acquire a number of energy
values that equals the number of light detecting elements in the column;
accumulating the number of energy values to provide a total energy value by
moving the light detecting elements in the column in a scan direction
serially past the pixel element and shifting in a direction opposite the
scan direction along the column the quantity of charge measured by the
light detecting element in present alignment with the pixel element to the
next adjacent light detecting element in the column; and
determining from the total energy value whether the amount of light present
in the pixel element represents a defect in the specimen subject.
15. Apparatus for detecting in a light pattern the presence of defects in a
specimen subject to which the light pattern corresponds,. comprising:
light detecting means for detecting light from the light pattern, the light
detecting means comprising light detecting elements arranged in a first
array of rows and columns having a first row and N total number of rows
and defining in the light pattern plural adjacent stripe regions each of
which includes plural pixel elements arranged in a second array of rows
and columns, and each light detecting element being operable to provide a
measured energy value corresponding to the amount of light present in any
one of the pixel elements;
scanning means to scan the light detecting means along a stripe region so
that each light detecting element in one column of the first array
traverses each pixel element in one column of the second array;
accumulating means to accumulate for each one of the pixel elements in the
one column of the second array a total energy value proportional to the
sum of the energy values acquired by all of the light detectign elements
in the one column of the first array;
position-detecting means for detecting the position of the first array
relative to the stripe region, the position-detecting means cooperating
with the accumulating means so that each one of the light detecting
elements in the first row of the one column never accumulates more than
one energy value for any one of the pixel elements of the second array
with which it becomes aligned, and each one of the light detecting
elements in the Nth row of the one column has accumulated N number of
energy values for any one of the pixel elements with which it becomes
aligned; and
means to determine from the total energy value for any one pixel element in
the one column whether the amount of light in the one pixel element
represents a defect in the specimen subject. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
This invention relates to the inspection for defects arising during the
fabrication of integrated circuits and, in particular, a method and an
apparatus for scanning a light pattern to detect the presence of defects
in a specimen subject to which the light pattern corresponds.
The use of light images in association with spatial filters has been
suggested heretofore for determining the presence of defects in a
photomask used in the fabrication of microminiature integrated circuits.
One system that purports to accomplish this task is described in U.S. Pat.
No. 4,000,949 of Watkins.
Light image constructed from holograms incorporated in optical processing
apparatus are especially beneficial in that reverse ray tracing techniques
can be employed during hologram reconstruction. Reverse ray tracing is
advantageous in eliminating aberrations which are introduced by the
optical components in the apparatus and which would spread out the light
rays representing defects in a photomask pattern that was exposed to form
the hologram. The spreading of light rays impairs the ability to detect
the defects. A system employing reverse ray tracing techniques is
described in U.S. Pat. No. 4,516,833 of Fusek.
The detection of defect image information in the light pattern can be
accomplished in accordance with the following procedure. A light sensitive
device having a defined imaging field is positioned to expose the device
to and measure the intensity of light present in the defect image region
of the size and shape of the imaging field and then is moved to the next
adjacent region to repeat the measurement process. The problem with a
detection procedure of this type is that relatively long settling times
are required after movement of the light sensitive device from one
measurement position to the next adjacent one. The reason is that the
alignment position of the light sensitive device relative to the defect
image region must be stable to ensure that detection of an image defect of
submicron dimensions can be effected. A detection system of this type is,
therefore, unacceptable in applications requiring relatively high
throughput defect detection rates (i.e., the rate at which an array type
photomask with normally identical elements can be inspected completely).
SUMMARY OF THE INVENTION
An object of this invention is to provide a method and an apparatus for
detecting in a light pattern the presence of defects in a specimen subject
to which the light pattern corresponds.
Another object of this invention is to provide such an apparatus that
continuously scans the light image pattern to detect at high throughput
rates defects of submicron dimensions in the specimen subject.
A further object of this invention is to provide in such an apparatus a
mechanism that coordinates the scan and exposure operation to shorten the
total time required to inspect the light pattern for defects in the
specimen subject.
Yet another object of this invention is to provide such a method that
promotes the detection of defect information of relatively low light
intensity during continuous scanning of the light image.
The method and apparatus of the present invention are described by way of
example with reference to a holographic imaging system that is useful for
inspecting photomasks and detecting defects in them. The imaging system
develops a defect image field whose dimensions and profile correspond to
the physical layout of the photomask under inspection. An inspection area
of the image field is partitioned into stripe regions of which each is
approximately 0.65 millimeters in width. The width of a stripe region
corresponds to the maximum extent of the image or optical window of a
light detecting means or camera that is employed to scan each stripe
region. The camera is a charge-coupled device that is mounted to an X-Y
positioning table, which moves the camera along the strip region in a
raster scan format. Control circuitry for the X-Y positioning table keeps
the camera moving at a nominally constant speed as it traverses each strip
region.
The determination of the presence of defects in the photomask is
accomplished by detecting the presence of regions of light in each stripe
region, which is divided into a first array of pixel elements. The
charge-coupled camera device comprises plural light detecting elements
that are arranged in a second array of 256 rows and 403 columns. In a
preferred embodiment of the invention, each light detecting element has a
rectangular aperture of 16 microns.times.20 microns. The size of the
second array dictates the size of the first array of pixel elements of
which each pixel element has dimensions in proportion to those of a light
detecting element. The number of columns of light detecting elements
equals the number of columns of pixel elements and dictates the width of a
stripe region. A lens that provides a tenfold magnification of the defect
image field equalizes the dimensions of the pixel elements and the light
detecting elements.
During a scan interval, the charge-coupled device continuously traverses
the columns of the first array in a direction along the length of the
strip region and acquires in row-by-row fashion measured energy values
which correspond to the intensities of light present in the pixel elements
with which the light detecting elements are aligned. In a preferred
embodiment, a quantity of charge which is developed in a light detecting
element of the charge-coupled device represents the measured energy value.
The energy value is proportional to the product of the quantity of charge
developed by the light intensity and the exposure time of the light
detecting element.
The quantity of charge measured for each pixel element in a row is shifted
serially along the corresponding column in synchronism with but in a
direction opposite to the motion of the camera device along the stripe
region. Each one of the light detecting elements in the last row of the
second array has a total charge value which is nominally equal to 256
times that of the pixel element to which it is aligned. Since the light
detecting elements do not have matched electrical characteristics, the
quantity of charge developed in response to the light present in a given
pixel element differs for different light detecting elements. The total
charge value in each light detecting element in the last row of the second
array represents, therefore, an average total charge for the corresponding
pixel element.
The total of the 256 energy values accumulated for each pixel element in
the last row of light detecting elements is converted by way of a video
amplifier and an analog-to-digital converter to digital format and is
transferred to a threshold detector which determines whether the quantity
of light indicates the presence of a defect in the photomask. Whenever the
charge-coupled device travels a distance of one row width along the stripe
region, 403 total charge values are processed by the threshold detector.
Whenever the threshold detector detects the presence of a defect, the
position coordinates of the X-Y positioning table, together with
information identifying the column of the pixel element and the total
charge value in digital form, are stored in a large digital memory unit.
After the camera completes a scan of the stripe region, the stage retraces
back to a different start location to begin a scan of the next adjacent
stripe region. Overscan borders at both the start and stop locations for
each stripe region provide regions for acceleration and deceleration of
the X-Y positioning table and thereby enable a continuous scan at a
nominally constant speed throughout the stripe region. This process is
repeated until the entire inspection area of the defect light image field
has been scanned.
The continuous scan of the defect image field by the charge-coupled device
camera during energy value acquisition promotes photomask inspection at
rapid throughput rates. The accumulation of only above-threshold
information in the defect image field, in combination with the continuous
scan capability, affords the rapid detection of defects of submicron
dimensions that are represented by regions of light of relatively low
intensity.
Additional objects and advantages of the present invention will be apparent
from the following detailed description of a preferred embodiment thereof,
which proceeds with refernce to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram of an array type photomask from which a light image
field of defect information is derived for inspection in accordance with
the present invention.
FIGS. 2A and 2B are diagrams of an optical processing system for,
respectively, constructing a hologram of a photomask pattern and
reconstructing from the hologram a defect light image corresponding to the
photomask.
FIG. 3 is an isometric view of the scanning mechanism for detecting in a
holographic light image field the presence of defects in the photomask of
FIG. 1.
FIG. 4A is an enlarged fragmentry view showing three strip regions in the
lower left-hand corner of the light image field of FIG. 3.
FIG. 4B is an enlarged, not to scale view of the stripe regions of FIGS. 3
and 4A that shows the raster scan path followed by a light detecting means
to detect defect images in the light image field in accordance with the
present invention.
FIG. 5 is a diagram showing the first array of pixel elements in the light
image field under tenfold magnification and the second array of light
detecting elements in the charge-coupled camera device used in the present
invention.
FIG. 6 is a block diagram of the apparatus for scanning the light image
field for defects in the photomask of FIG. 1 in accordance with the
present invention.
FIG. 7 is a flow diagram that shows the defect image processing carried out
by a general purpose computer included in the apparatus of FIG. 6.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENT
FIG. 1 is a diagram of a photomask 10 which comprises a transparency, such
as a chrome coated glass plate, that includes a planar array of normally
identical photographic image elements 12 mutually spaced apart by a
distance 14. Each image element 12 defines a circuit pattern which
corresponds to a layer of an integrated circuit device that is fabricated
in part with the use of the photomask. Photomask 10 is of rectangular
shape with sides 16a, 16b, 16c, and 16d. The number of image elements 12
varies typically from between 20 and 500, although fewer or greater
numbers are found in special instances. A noteworthy type of photomask
which is called a reticle has only one image element.
FIGS. 2A and 2B show optical processing apparatus for, respectively,
constructing a hologram of photomask 10 and developing from the hologram a
defect image corresponding to the photomask. The optical processing
practiced with the use of such apparatus for inspecting photomasks for
defects is described in the Fusek patent.
With reference to FIG. 2A, a subject beam 18 illuminates photomask 10 which
diffracts the light rays incident to it. The diffracted light rays 20 pass
through an imaging lens 22 and interfere with a reference beam 24 at a
location beyond the Fourier transform plane 26 in the back focal plane of
the imaging lens. The light rays 20 and the reference beam 24 interfere in
a photographic recording medium to form a hologram 28. The hologram 28
records light information relating to the pattern of photomask 10 and the
defects present in it.
With reference to FIG. 2B, a three-dimensional image 10' of photomask 10 is
reconstructed from hologram 28 in the same optical processing system,
which also includes a spatial filter 30 that conforms to the Fourier
transform of an error-free photomask reference pattern. Spatial filter 30
is positioned in Fourier transform plane 26. Hologram 28 is illuminated by
a light beam 24' that is coherent to but propagates in the conjugate
direction of reference beam 24. Spatial filter 30 eliminates the light
information corresponding to the photomask pattern from the reconstructed
holographic light pattern. The light information or rays 20' not blocked
by spatial filter 30 are reimaged by imaging lens 22 and represent the
defects present in the photomask pattern. The holographic image 10'
constitutes regions of relatively low intensity light. The holographic
image 10' can describe photomask defects of submicron dimensions of about
0.5 micron. The light regions in defect image 10' appear in the same
relative positions as the corresponding defects appear on photomask 10.
FIG. 3 is a diagram of a scanning mechanism for detecting in a holographic
defect image 10' the presence of defects in photomask 10 to which the
image corresponds. A defect image field 32 represents the region in the
optical processing system in which statonary aerial light images 10' of
relatively low intensity appear. Defect image field 32 is defined by sides
34a, 34b, 34c, and 34c, which correspond to the respective sides 16a, 16b,
16c, and 16d of, but do not necessarily have the same absolute dimensions
as, photomask 10. In the preferred embodiment, a lens 35, which is shown
in phantom, provides a tenfold magnification of the defect images 10'
present in defect image field 32 relative to the size of photomask 10 for
reaosns that will be described below.
The presence of defects in photomask 10 is determined by detecting within
an inspection area 36 of defect image field 32 regions of light of
intensities that exceed a predetermined threshold amount. Inspection area
36 includes the space contained within the outline 38, which is defined by
the next adjacent sides of image elements 12 to the sides of photomask 10
in FIG. 1.
A defect image 10' that appears within image field 32 is formed by light
rays 20' that converge in a horizontal plane 40. Lens 35 produces beneath
plane 40 a defect image of tenfold magnification of that appearing in
plane 40. The detection of defects is accomplished by scanning inspection
area 36 in a raster type format with a light detecting means or camera 42
to sense the presence of light. The scanning procedure takes place within
a plane 44 which is parallel to and beneath plane 40 and lens 35. In a
preferred embodiment, camera 42 is solid state imaging device that
comprises a charge-coupled device. One device of this type is a model
SID504 chargecoupled device manfactured by RCA, New Products Division,
Lancaster, PA 17604-3140, USA. The reference made herein to the design and
architecture of the SID504 device are for purposes of illustration only.
Camera 42 and lens 35 are supported on a plate 46 that comprises a portion
of an X-Y positioning table 48 which is cable of positioning camera 42 in
plane 44 for alignment with any location in defect image field 32. A top
of Y-stage 50 of positioning table 48 supports plate 46 and moves camera
42 along the Y direction in plane 44. A bottom or X-stage 52 of
positioning table 48 moves camera 42 along the X direction in plane 44.
One suitable type of X-Y positioning table is a model 8500 manufactured by
Kensington Laboratories, Inc. of Richmond, Calif.
FIGS. 4A and 4B are diagrams of, respectively, the outline of the lower
left-hand corner of inspection area 36 in FIG. 3 and an enlarged portion
thereof to show the raster scan path of camera 42. FIG. 5, which is an
enlarged diagram of the portions of the stripe regions of FIG. 4B, shows
the one-to-one correspondence between the dimensions of the light
detecting elements of the second array and the pixel elements of the first
array resulting from the tenfold magnification by lens 35.
With reference to FIGS. 3, 4A, 4B, and 5, camera 42 has an otpical window
54 through which light passes to be detected by it. Optical window 54 is a
rectangle which has sides 56 and 58 that define its length and sides 60
and 62 that define its width.
In a normal scan operation, positioning table 48 moves camera 42 along a
scan path segment 64a in the X direction from a start location 66 so that
side 62 of optical window 54 is aligned with the segment 68 of inspection
area 36. Sides 56 and 58 of optical window 54 are parallel to the Y
direction and define the width 70 of a stripe region 72 (three of which
are shown in FIG. 3) which represents the portion of inspection area 36
traversed by camera 42 as it moves in the X direction.
After side 56 of optical window 54 moves past segment 74 of inspection area
36, positioning table 48 moves camera 42 along a retrace path segment 76a
to a start location 78 to scan a second adjacent stripe region 72. During
retrace, Y-stage 50 moves camera 42 at a distance equal to width 70 of
strip region 72, and X-stage 52 moves camera 42 a distance equal to the
length of path segment 64a. After retrace, positioning table 48 moves
camera 42 along path segment 64b in the X direction from start location 78
to traverse a second strip region 72 of width 70.
The above-described scanning and retrace procedure is repeated until the
entire inspection area 36 is traversed by camera 42. There are, however,
differences in the lengths of the scan and retrace path segments to
accommodate the difference in the dimensions in the X direction of
inspection area 36.
With particular reference to FIG. 5, camera 42 of the preferred embodiment
is an RCA model SID504 charge-coupled device that includes an array 82 of
light detecting elemetns 84 arranged in rows 86 and columns 88. Array 82
has 256 rows and 403 columns of light detecting elements 84. A row 86 is
defined as a group of elements 84 arranged in a line perpendicular to the
scan direction (i.e., in the Y direction), and a column 88 is defined as a
group of elements 84 arranged in a line parallel to the scan direction
(i.e., in the X direction). Each row 86 and each column 88 have lengths of
6.5 millimeters and 4.9 millimeters, respectively. Each light detecting
element 84 is 20 microns in length and 16 microns in width. The width of
each one of stripe regions 72 is, therefore, equal to the total distance
spanned by a row of 403 light detecting elements. Each one of light
detecting elements 84 receives through optical window 54 light rays that
emanate from the portion of inspection area 36 with which it is aligned
and stores in its potential well a quantity of charge or measured energy
value that corresponds to the intensity of the light rays incident to it.
Each stripe region 72 of inspection area 36 is divided into an array 90 of
pixel elements 92, of which each has the same dimensions as light
detecting elements 84 of array 82 by the operation of lens 35. Pixel
elements 94 of array 90 are arranged in rows 94 and columns 96, each row
having 403 pixel elements and each column having a number of pixel
elements dictated by the length of the stripe region 72. The presence of
light in the stripe regions is detected by moving optical window 54 of
camera 42 along each one of stripe regions 72 and acquiring the energy
value corresponding to the intensity of light in each one of pixel
elements 92 in accordance with the following procedure.
X-stage 52 commences the scanning process by accelerating optical window 54
from start location 66 toward the right in the X direction until its side
58 is collinear with segment 98 of inspection area 36. X-stage 52 then
moves optical window 54 at a nominally constant predetermined speed along
stripe region 72.
Whenever light detecting elements 84 in the first row 86a of array 82 align
with pixel elements 92 in the first row 94a of array 90, the following
events take place. An electrical charge develops in the potential well of
each one of light detecting elements 84 in row 86a. The quantity of charge
corresponds to the intensity of light present in the pixel element. (The
potential wells of light detecting elements 84 have no charge accumulated
in them prior to the scan of a stripe region 72.) A ROW TRANSFER clock
signal that is applied to each row 86 of array 82 transfers the charge
from each light detecting element 84 in row 86a to the light detecting
element in the same column 88 but in the next adjacent or second row 86b.
This transfer takes place about the time the light detecting elements and
the pixel elements are aligned with each other. (Since X-stage 52
continuously moves optical window 54 along stripe region 72, there is a
negligible amount of image degradation that results from aliasing between
adjacent rows of the pixel elements.) After the transfer of charge from
row 86a to row 86b, there exists no accumulated charge in the potential
wells of light detecting elements 84 in row 86a.
Whenever light detecting elements 84 in second row 86b align with pixel
elements 92 in the second row 94b of array 90, the following events take
place. An electrical charge develops in the potential well of each light
detecting element 84 in rows 86a and 86b. The quantity of charge developed
in each one of the light detecting elements 84 in row 86b is added to the
charge previously tranferred to it. The quantity of charge in the light
detecting elements 84 in row 86b represents, therefore, two energy values
corresponding to the intensity of light present in a pixel element 92 in
each column of row 94a of array 90. The ROW TRANSFER clock signal
transfers the charge from each light detecting element 84 in row 86b and
row 86a to the light detecting element in the same column 88 but in the
next adjacent third row 86c and second row 86b, respectively.
The above-described procedure of (1) acquiring in a light detecting element
84 in a row 86 an energy value corresponding to the intensity of light in
a pixel element 94 with which the light detecting element is aligned and
(2) transferring the energy value to the light detecting element 84 in the
same column 88 but in the next adjacent row 86 with which the pixel
element 92 has not previously been aligned is repeated for 255 cycles of
the ROW TRANSFER clock signal.
Whenever 255 such row-to-row transfers have been completed, the light
detecting elements in the 256th or last row 86d of array 82 align with the
pixel elements 92 in first row 94a of array 90. The 255 previously
accumulated energy values for each pixel element 92 in first row 94a are
added to the 256th energy value acquired by each light detecting element
84 in last row 86d. Prior to the occurrence of the 256th ROW TRANSFER
clock signal, energy values accumulated in the 403 light detecting
elements 84 corresponding to the pixel elements 92 in row 94a are read out
serially by a high-speed DATA TRANSFER clock signal. The accumulated
energy values for pixel elements 94 are converted to a digital format and
processed by a threshold detector to determine whether the amount of light
present in each pixel element 92 indicates the presence of a defect in a
corresponding location in photomask 10.
Upon the occurrence of the 256th cycle of the ROW TRANSFER block signal,
the 255 previously accumulated energy values for each pixel element 94 in
second row 94b are added to the 256th energy value acquired by each light
detecting element 84 in last row 86d. Prior to the occurrence of the 257th
cycle of the ROW TRANSFER clock signal, the contents of the 403 light
detecting elements 84 corresponding to the pixel elements 92 in row 94b
are read out and processed as described above.
For each succeeding cycle of the ROW TRANSFER clock signal, the scan of
stripe region 72 continues such that 256 energy values for each pixel
element 92 in a row 94 and a column 96 of array 90 are accumulated in the
light detecting element 84 in the corresponding column 88 and row 86d of
array 82.
There are several general aspects of the accumulation of energy values that
characterize the scanning process of the present invention. First, each
one of the light detecting elements 84 in row 86a never accumulates more
than one energy value for any one of the pixel elements 92 with which it
becomes aligned. Second, the light detecting elements 84 in a row 86
presently aligned with the pixel elements 92 in a particular row 94 always
have one more energy value accumulated in them than the light detecting
elements 84 in the next adjacent row 86 that was previously aligned with
the particular row 94 of pixel elements 92. Third, each one of the light
detecting elements 84 in row 86d accumulates 256 energy values
corresponding to the light present in the pixel element 92 with which it
is aligned.
After side 56 of optical window 54 travels past segment 74 of inspection
area 36, the scan of a stripe region 72 is completed, and the accumulated
energy values of the pixel elements 92 in the last row 94d of array 90
have been read out from the light detecting elements 84 of the last row
86d of array 82. X-stage 52 decelerates optical window 54 to a stop at
stop location 100. (In FIG. 5, optical window 54 is shown in phantom in
this position.) X-stage 52 and Y-stage 50 retrace camera 42 along path
segment 76a to position optical window 54 at start location 78. The
potential wells of light detecting elements 84 are cleared during this
time in preparation for the scan of the next adjacent stripe region 72.
The scan and retrace of the second and succeeding stripe regions 72
proceed as described above.
FIG. 6 is a block diagram of the apparatus for scanning a holographic
defect image for defects in accordance with the present invention. FIG. 7
is a flow diagram showing the interrelationship of the various functions
carried out by a general purpose computer included in the apparatus of
FIG. 6. The description below of the operation of the scanning apparatus
of FIG. 6 is presented with parenthetical references to the appropriate
process or decision blocks in the flow diagram of FIG. 7.
With reference to FIGS. 6 and 7, the scanning apparatus 200 emplo | | |
