U.S. patent application number 11/020603 was filed with the patent office on 2005-05-26 for scanning system for inspecting anamolies on surfaces.
Invention is credited to Leslie, Brian C., Nikoonahad, Mehrdad, Stokowski, Stanley E., Wells, Keith B..
Application Number | 20050110986 11/020603 |
Document ID | / |
Family ID | 31996994 |
Filed Date | 2005-05-26 |
United States Patent
Application |
20050110986 |
Kind Code |
A1 |
Nikoonahad, Mehrdad ; et
al. |
May 26, 2005 |
Scanning system for inspecting anamolies on surfaces
Abstract
An optical scanning system and method for detecting anomalies,
including pattern defects and particulate contaminants, on both
patterned and unpatterned surfaces, using a light beam, scanning at
a grazing angle with respect to the surfaces, a plurality of
detectors and an interchannel communication scheme to compare data
from each detector, which facilitates characterizing anomalies. The
light beam illuminates a spot on the surface which is scanned over
a short scan-line. The surface is moved in a manner so that the
spot is scanned over its entire area in a serpentine fashion along
adjacent striped regions. The plurality of detectors include groups
of collector channels disposed circumferentially around the
surface, a bright field reflectivity/autoposition channel, an
alignment/registration channel and an imaging channel. The
collector channels in each group are symmetrically disposed, in the
azimuth, on opposite sides of the center of the scan line. The
position of the collector channels, as well as the polarization of
the beam, facilitates distinguishing pattern defects from
particulate contaminants. The bright field
reflectivity/autoposition channel is positioned to receive
specularly reflected light that carries information concerning
local variation in reflectivity, which is used to classify detected
anomalies, as well as determine variations in the height of the
surface. The alignment/registration channel is positioned to detect
a maximum of the light scattered from the pattern on the surface to
ensure that the streets of die present on the surface are oriented
so as not to be oblique with respect to the scan line. The imaging
channel combines the advantages of a scanning system and an imaging
system while improving signal/background ratio of the present
system.
Inventors: |
Nikoonahad, Mehrdad; (Menlo
Park, CA) ; Stokowski, Stanley E.; (Danville, CA)
; Wells, Keith B.; (Santa Cruz, CA) ; Leslie,
Brian C.; (Cupertino, CA) |
Correspondence
Address: |
PARSONS HSUE & DE RUNTZ LLP
655 MONTGOMERY STREET
SUITE 1800
SAN FRANCISCO
CA
94111
US
|
Family ID: |
31996994 |
Appl. No.: |
11/020603 |
Filed: |
December 21, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11020603 |
Dec 21, 2004 |
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09954287 |
Sep 11, 2001 |
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09954287 |
Sep 11, 2001 |
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09760558 |
Jan 16, 2001 |
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6636302 |
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09760558 |
Jan 16, 2001 |
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09213022 |
Dec 16, 1998 |
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6215551 |
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09213022 |
Dec 16, 1998 |
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08499995 |
Jul 10, 1995 |
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5883710 |
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08499995 |
Jul 10, 1995 |
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08351664 |
Dec 8, 1994 |
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Current U.S.
Class: |
356/237.2 |
Current CPC
Class: |
G01N 21/9501 20130101;
G01N 21/956 20130101; G01N 21/94 20130101 |
Class at
Publication: |
356/237.2 |
International
Class: |
G01N 021/88 |
Claims
1-52. (canceled)
53. An optical scanning system for detection of anomalies, such as
particles, on a surface comprising: means for producing a beam of
light, means for directing the beam onto a sample surface of the
type having locations with periodic and non-periodic features,
producing an illuminated spot thereon with the beam having a
grazing angle of incidence, means for scanning the spot, means for
detecting light scattered from the surface, including a plurality
of detectors symmetrically positioned about the surface, to collect
laterally scattered light, defining a first group of detectors,
with each of the plurality of detectors producing a first
electrical signal representing scattered light; and means, in
electrical communication with the opposed detectors, for processing
each first electrical signal independently of the other to
determine the presence or absence of anomalies, the processing
means including a means for producing a map from each first
electrical signal, defining a first map set, the first map set
representing the entire scan path over the surface, the processing
means including a means for comparing maps of the first map set to
identify anomalies.
54. The optical scanning system as recited in claim 53 wherein the
scanning means scans the spot in a serpentine fashion over the
sample surface.
55. The optical scanning system as recited in claim 53 wherein the
detecting means includes a second group of a plurality of detectors
symmetrically positioned about the surface, to collect forwardly
scattered light, with each of the plurality of detectors in the
second group producing a second electrical signal representing
scattered light.
56. The optical scanning system as recited in claim 55 wherein the
producing means forms a second map set from the second electrical
signals, representing the entire scan path, with the comparing
means comparing maps of the second map set to identify
anomalies.
57. The optical scanning system as recited in claim 56 wherein the
classifying means produces a third signal representing the presence
of pattern defects and a fourth signal representing particulate
contaminants, based upon scattered light information carried by the
first and second map set.
58. The optical scanning system as recited in claim 55 wherein each
collection channel includes a variable aperture to optimize the
collection angle for detecting scattered light.
59. The optical scanning system as recited in claim 55 wherein each
collection channel includes a variable polarization filter to allow
light having desired predetermined states of polarization to pass
therethrough, while attenuating light that does not have the
desired state of polarization, and a polarizing element placed in
the path of the beam, between the scanning means and the surface,
to polarize the beam to a predetermined state of polarization,
wherein each collection channel detects an anomaly selected from
the group consisting of particulate contaminant or pattern
defect.
60. The scanning system as recited in claim 59 wherein the
polarizing element is of the type to provide an S polarization
state of the beam passing therethrough.
61. The scanning system as recited in claim 59 wherein the
polarizing element is of the type to provide a P polarization state
of the beam passing therethrough.
62. The scanning system as recited in claim 59 wherein the
polarizing element is of the type to provide a left handed circular
polarization state of the beam passing therethrough.
63. The scanning system as recited in claim 59 wherein the
polarizing element is of the type to provide a right handed
circular polarization state of the beam passing therethrough.
64. The scanning system as recited in claim 60 wherein the variable
filters associated with the first group of detectors attenuates
polarized scattered light with the variable filters associated with
the second group of detectors attenuating scattered light which is
not in a P state of polarization.
65. The scanning system as recited in claim 61 wherein the variable
filters associated with the first group of detectors attenuates
polarized scattered light with the variable filters associated with
the second group of detectors attenuating scattered light which is
not in an S state of polarization.
66. The scanning system as recited in claim 60 wherein the
plurality of variable filters attenuates scattered light not having
an S state of polarization.
67. The scanning system as recited in claim 60 wherein the
plurality of variable filters attenuates scattered light not having
a P state of polarization.
68. The scanning system as recited in claim 61 wherein the
plurality of variable filters attenuates scattered light not having
an S state of polarization.
69. The scanning system as recited in claim 61 wherein the
plurality of variable filters attenuates scattered light not having
a P state of polarization.
70. The optical scanning system as recited in claim 53 wherein the
first group of detectors are a pair of opposed detectors oriented
to receive light scattered in a direction less than 30 degrees
above the surface and symmetrically disposed on opposite sides of
the scanning beam at an azimuthal angle of 75 to 95 degrees, with
respect to the scanning beam.
71. The optical scanning system as recited in claim 54 wherein the
second group of detectors are a pair of spaced-apart detectors
oriented to receive light scattered in a direction less than 30
degrees above the surface arid symmetrically disposed on opposite
sides of the scanning beam at an azimuthal angle of 30 to 60
degrees, with respect to the scanning system.
72. The optical scanning system as recited in claim 53 further
including an autoposition detector for collecting specularly
reflected light from the spot, the autoposition detector having a
means for measuring a change in height of the surface.
73. The optical scanning system as recited in claim 72 wherein the
autoposition detector includes a reflectivity channel producing a
normalization signal representing a threshold voltage based upon
the intensity of reflecting light, wherein those anomalies
represented by a signal having a voltage level less than the
threshold voltage are discarded.
74. The optical scanning system as recited in claim 53 including a
normal detector positioned normal to said surface to collect
upwardly scattered light from the spot.
75. The optical scanning system as recited in claim 74 wherein the
normal detector includes an array of sensors for forming pixels
consisting of a plurality of electronic bit signals corresponding
to the image viewed by the sensors, and means for transferring a
charge from each said pixel in synchronism with the scanning spot
so that each pixel receives light scattered from a unique area
illuminated by the spot along the scan line.
76. The optical scanning system as recited in claim 53 wherein the
features include a plurality of dies each having streets forming a
grid and further including an alignment detector positioned above
said surface to maximize collection of light scattered from the
features on the surface, the alignment detector being connected to
a means for aligning the surface so that the scan line is not
oblique to the streets of the die.
77. The optical scanning system as recited in claim 56 further
including an autoposition detector, collecting specularly reflected
light, and a normal detector positioned normal to the surface,
collecting upwardly scattered light, the autoposition detector
producing a normalization signal representing selected light and
the normal detector producing a third electrical signal
representing upwardly scattered light, with the producing means
forming a third map from the normalization signal and a fourth map
from the third electrical signal, whereby the producing means
performs logical operations, comparing the maps to detect and
characterize anomalies, on the surface.
78. The optical scanning system as recited in claim 77 wherein the
logical operations are digital operations from the group consisting
of AND, OR and XOR.
79. An optical scanning system for detection of anomalies on a
surface comprising: means for producing a beam of light, means for
directing the beam onto a sample surface of the type having
locations with periodic and non-periodic features, producing an
illuminated spot thereon with the beam having a grazing angle of
incidence, means for scanning said spot, in a serpentine fashion,
over the sample surface, each scan defining a short scan line,
means for detecting light scattered from the surface, including a
pair of opposed collection channels symmetrically positioned about
the surface, on opposite sides of the beam to collect laterally
scattered light and a pair of spaced apart collection channels
symmetrically positioned about the surface, on opposite sides of
the beam to collect forwardly scattered light with the opposed
collection channels producing a first and a second electrical
signal and the spaced apart collection channels producing a third
and a fourth electrical signals, each electrical signal
representing scattered light impinging on a collection channel, and
means, in electrical communication with each collection channel,
for processing each of the signals therefrom independently of the
remaining signals to determine the presence or absence of
anomalies.
80. The optical scanning system as recited in claim 79 wherein the
opposed collection channels are positioned on opposite sides of the
beam of an azimuthal angle in the range of 75-95 degrees, and the
spaced apart channels are positioned on opposite sides of the beam
at an azimuthal angle in the range of 30-60 degrees.
81. The optical scanning system as recited in claim 79 wherein the
processing means includes a means for producing a map from each
electrical signal representing the entire scan path over the
surface with a first map generated from the first signal, the
second map generated from the second signal, the third map
generated from the third signal and the fourth map generated from
the fourth signal, the processing means including a means for
comparing the first map with the second map, with the differences
between them being anomalies recorded as a first composite map, and
comparing the third map with the fourth map, with the differences
between them being anomalies recorded as a second composite
map.
82. The optical scanning system as recited in claim 81 wherein the
processing means further includes a classifying means for comparing
the first composite map with the second composite map, thereby
classifying the anomalies present as a material selected from the
group consisting of particulate contaminant or pattern defect.
83. The optical scanning system as recited in claim 82 including a
normal detector positioned normal to said surface to collect
upwardly scattered light from the spot.
84. The optical scanning system as recited in claim 83 wherein the
normal detector includes an array of sensors for forming pixels
consisting of a plurality of electronic bit signals corresponding
to the image viewed by the sensors, and means for transferring a
charge from each of the pixels in synchronism with the scanning
spot so that each pixel receives light scattered from a unique area
illuminated by the spot along the scan line.
85. The optical scanning system as recited in claim 84 wherein each
collection channel includes a variable aperture to optimize the
collection angle for detecting scattered light.
86. The optical scanning system as recited in claim 85 wherein each
collection channel includes a variable polarization filter, each of
the variable filters being adjusted to permit detection of light
having a state of predetermined polarization, and a polarizing
element placed in the path of the beam, between the producing means
and the surface to place the beam in a predetermined state of
polarization, wherein each collection channel detects an anomaly
selected from the group consisting of particulate contaminant and
pattern defect.
87. An optical scanning system for detection of anomalies on a
surface comprising: means for producing a beam of light, means for
directing the beam onto a sample surface, producing an illuminated
spot thereon, means in the path of the light beam for scanning said
spot along a scan line, a detector having an array of sensors for
forming pixels consisting of a plurality of electronic bit signals
corresponding to the image viewed by the array of sensors, and
means for transferring a charge from each of the pixels in
synchronism with the scanning spot so that each pixel receives
light scattered by the spot along the scan line, producing an
electrical signal representing scattered light, and means, in
electrical communication with the detector, for processing the
electrical signals received from the sensors, producing an
image.
88. The optical scanning system as recited in claim 87 wherein the
array of sensors are positioned normal to the sample surface for
collecting upwardly scattered light therefrom.
89. The optical scanning system as recited in claim 87 wherein each
pixel is positioned so as to receive light scattered from a unique
area of the sample surface, illuminated by the spot along the scan
line.
90. A method of detecting anomalies on a surface comprising:
scanning an illuminated spot over a substantially flat surface of
the type having locations with periodic and non-periodic features,
simultaneously detecting laterally scattered light on opposite
sides of the scanning spot at an azimuthal angle in the range 75-95
degrees, with respect to the scanning spot, producing a first and a
second electrical signal representing scattered light, producing a
first map from the first signal, representing light detected along
the entire scan path and a second map from the second signal,
representing light detected along the entire scan path, and
comparing the first map with the second map, differences between
them representing anomalies on the surface, defining a first
composite map.
91. The method as recited in claim 90 further including the steps
of simultaneously detecting forwardly scattered light on opposite
sides of the scanning spot at an azimuthal angle in the range of
30-60 degrees, producing third and fourth electrical signal,
producing a third map from the third signal, representing light
detected along the entire scan path and a fourth map from the
fourth signal, representing light detected along the entire scan
path, and comparing the third map with the fourth map, differences
between them representing anomalies on the surface, defining a
second composite map.
92. The method as recited in claim 91 further including the step of
classifying the anomalies by comparing the first composite map with
the second composite map, wherein each anomaly detected is
classified as a material selected from the group consisting of
particulate contaminant or pattern defect.
93. The method as recited in claim 90 further including the steps
of, detecting specularly reflected light and producing a
normalization signal having a threshold voltage level based upon
the intensity of detected reflected light, and normalizing the
differences found by comparing the voltage level of the first,
second, third and fourth signals with the threshold level, wherein
anomalies represented by a signal having a voltage level less than
the threshold level are discarded.
94. The method as recited in claim 87 further including the step of
moving the surface so that it is scanned in a serpentine fashion
over the entire surface.
95. The method as recited in claim 91 wherein both laterally and
forwardly scattered light is detected at an angle of elevation of
less than 30 degrees, with respect to the surface.
96. The method as recited in claim 91 wherein the features include
a plurality of dies each of which has a plurality of streets
positioned at rights angles to one another and further including
the step of, aligning the surface so that a scan line is not
oblique with respect to the streets.
97. A method of detecting anomalies on a surface comprising:
scanning an illuminated spot over a surface of, the type having
locations with periodic and non-periodic features, simultaneously
detecting laterally scattered light, forwardly scattered light,
upwardly scattered light and specularly reflected light with an
optical detecting system, and producing a first plurality of
electrical signals, representing laterally scattered light, a
second plurality of electrical signals, representing forwardly
scattered light, a third signal representing upwardly scattered
light and a normalization signal representing specularly reflected
light, producing a first map from the first signal, a second map
from the second signal, a third map from the third signal and a
fourth map from the normalization signal with each map representing
light detected along the entire scan path, and comparing the first
map, second map, third map and fourth map to determine the presence
and type of anomaly.
98. The method as recited in claim 97 wherein the normalization
signal defines a threshold voltage and further including the step
of comparing the voltage level of the first, second, third and
fourth signals with the normalization signal, wherein anomalies
represented by a signal having a voltage level less than the
threshold are discarded.
99. The method as recited in claim 97 wherein the comparing step
includes performing logical operations on data represented by the
first, second, third and fourth maps, the logical operations
selected from the group consisting of AND, OR and XOR.
100. The method as recited in claim 97 wherein laterally scattered
light is detected at the same azimuthal angle, simultaneously on
opposite sides on the scanning spot with the azimuthal angle being
in the range 75-95 degrees, with respect to the scanning spot, and
the forwardly scattered light is detected at the same azimuthal
angle, simultaneously on opposite sides on the scanning spot, with
the azimuthal angle being in the range 30-60 degrees, with respect
to the scanning spot.
101. The method as recited in claim 100 wherein laterally and
forwardly, scattered light is detected at an angle of elevation of
less than 30 degrees, with respect to the surface.
102. The method as recited in claim 97 wherein the upwardly
scattered light is detected by an array of sensors for forming
pixels consisting of a plurality of electronic bit signal
corresponding the an image viewed by the sensor and further
including the step transferring a charge from each pixel in
synchronism with the scanning of that spot so that each pixel
receives light scattered from a unique area illuminated by the spot
along the scan line.
103. The method as recited in claim 97 further including the step
of producing a reflectivity map of the surface based upon the
information carried by the normalization signal.
104. A surface inspection method for distinguishing between
particles on a surface and defects in the surface, comprising:
receiving the surface; causing the surface to be scanned by a
P-polarized beam of light at an oblique angle to the surface;
collecting light scattered from the surface at a first imaging
channel, and at least at a second oblique channel, said second
channel offset angularly from said first imaging channel;
converting the collected light from the two channels into
respective signals representative of light scattered into the two
channels; comparing the two signals and determining whether a
defect is one of a particle or a defect in the surface based at
least in part on said comparing.
105. The method of claim 104, wherein said second channel is offset
at least forwardly of said first channel.
106. The method of claim 105, wherein said second channel receives
substantially only forward scattered light.
107. The method of claim 104, wherein said first channel includes a
scattering direction substantially perpendicular to the
surface.
108. The method of claims 104-107, wherein said signals are
representative of intensities of the light scattered into said
channels.
109. The method of claim 104-107, wherein said P-polarized light is
incident upon said surface at an angle in a range of about 55 to 85
degrees of perpendicular.
110. The method of claim 104-107, wherein the light collected in
said second channel is scattered at an angle in the range of about
3 to 30 degrees from the surface.
111. The method of claim 104, additionally including forming a
first display map identifying the locations of surface defects on
the surface.
112. The method of claim 111, including forming a second display
map identifying the locations of particle defects on the
surface.
113. The method of claim 104, wherein the scanning further
comprises translationally transporting the surface along a
path.
114. A surface inspection method for distinguishing between
particles on a surface and defects in the surface, comprising:
receiving the surface at a surface inspection system; scanning the
surface at the inspection system with a beam of P-polarized light
at an angle of incidence oblique to the surface; collecting light
scattered from the surface at the inspection system at a first
central zone, and at least at a second oblique zone offset
angularly from said first zone; converting the collected light
components from said zones into respective signals representative
of light scattered into said zones; comparing said signals and
determining whether a defect is one of a particle or a defect in
the surface based at least in part on said comparing.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 09/954,287, filed Sep. 11, 2001, which is a
continuation of U.S. patent application Ser. No. 09/760,558, filed
Jan. 16, 2001, now U.S. Pat. No. 6,636,302, which is a continuation
of U.S. patent application Ser. No. 09/213,022, filed Dec. 16,
1998, now U.S. Pat. No. 6,215,551, which is a continuation of U.S.
patent application Ser. No. 08/499,995, filed Jul. 10, 1995, now
U.S. Pat. No. 5,883,710, which is a continuation-in-part
application of parent application entitled "Optical Scanning System
for Surface Inspection," by Mehrdad Nikoonahad, Keith B D. Wells
and Brian C. Leslie, Ser. No. 08/351,664, filed Dec. 8, 1994, now
abandoned. This application is also related to the patent
application entitled "Optical Wafer Positioning System," by Mehrdad
Nikoonahad, Philip R. Rigg, Keith B. Wells and David S. Calhoun,
Ser. No. 08/361,131, filed Dec. 21, 1994 ("Related Application"),
which has since issued as U.S. Pat. No. 5,530,550. Both prior
applications are incorporated by reference herein in their
entirety.
BACKGROUND OF THE INVENTION
[0002] This invention relates in general to surface inspection
systems, and in particular, to a high speed scanner system for
inspecting anamolies on surfaces such as semiconductor wafers,
photomasks, reticles, ceramic tiles, and other surfaces.
[0003] The size of semiconductor devices fabricated on silicon
wafers has been continually reduced. At the time this application
is filed, for example, semiconductor devices can be fabricated at a
resolution of a half micron or less and sixty-four (64) megabyte
DRAMs are being fabricated with 0.35 micron design rule. The
shrinking of semiconductor devices to smaller and smaller sizes has
imposed a much more stringent requirement for sensitivity of wafer
inspection instruments which are called upon to detect contaminant
particles and pattern defects that are small compared to the size
of the semiconductor devices. On the other hand, it is desirable
for wafer inspection systems to provide an adequate throughput so
that these systems can be used in production runs to detect
defective wafers.
[0004] In U.S. Pat. No. 4,898,471 to Stonestrom et al. assigned to
the present assignee to the present application, the area
illuminated on a wafer surface by a scanning beam is an ellipse
which moves in the scan direction. In one example given by
Stonestrom et al., the ellipse has a width of 20 microns and a
length of 115 microns. Light scattered by anomalies or patterns in
such illuminated area is detected by photodetectors placed at
azimuthal angles in the range of 80 to 100.degree. The signals
detected by the photodetectors are used to construct templates.
When the elliptical spot is moved in the scan direction to a
neighboring position, scattered light from structures within the
spot is again detected and the photodetector signal is then
compared to the template to ascertain the presence of contaminant
particles or pattern defects as opposed to regular pattern. In
Stonestrom et al., the scanning beam scans across the entire wafer
to illuminate and inspect a long narrow strip of the wafer
extending across the entire dimension of the wafer in the scanning
direction. The wafer is then moved by a mechanical stage in a
direction perpendicular to the scanning direction for scanning a
neighboring elongated strip. This operation is then repeated until
the entire wafer is covered.
[0005] While the system of Stonestrom et al. performs well for
inspecting wafers having semiconductor devices that are fabricated
with coarser resolution, with the continual shrinking of the size
of the devices fabricated, it is now desirable to provide an
improved inspection tool that can be used to detect very small size
anomalies that may be difficult to detect using Stonestrom et al.'s
system.
SUMMARY OF THE INVENTION
[0006] This invention is based on the recognition that very small
anamolies can be detected by reducing the size of the area that is
illuminated by the scanning light beam. Light scattered from
structures in the spot will include background, such as light
scattered by pattern on the surface, as well as light that is
scattered by anomalies such as contaminant particles, pattern
defects or imperfections of the surface. Such background can have a
significant amplitude. For this reason, if the anamoly is of a size
which is small compared to the size of the illuminated area, the
scattered light from such anamoly may be overwhelmed by and become
undetectable from the background. By reducing the size of the
illuminated area or spot size, the ratio of the light intensity
scattered by an anomaly to that of the background will be
increased, thereby increasing detection sensitivity. However, with
a smaller spot size, it will be more difficult to maintain the
uniformity of the spot along a long straight scan line across the
entire wafer. By breaking up the scan path into short segments, it
is possible to employ a smaller spot size while at the same time
maintaining uniformity of the spot along the path. From the system
point of view, by reducing the length of the scan, the size of the
collection optics for detecting forward scattered light becomes
more manageable.
[0007] Thus one aspect of the invention is directed towards a
method for detecting anamolies on a surface, comprising the steps
of directing a beam of light at a grazing angle towards the
surface, causing relative motion between the beam and the surface
so that the beam scans a scan path covering substantially the
entire surface; and collecting light scattered along said path for
detecting anamolies. The scan path includes a plurality of arrays
of scan path segments, wherein each of at least some of such scan
path segments has a span shorter than the dimensions of the
surface.
[0008] As used in this application, "minimum width" of the
illuminated area or spot on the surface to be inspected is defined
as the minimum dimension of a boundary around the area or spot
along any direction on the surface, where the boundary is defined
as the locations on the surface where the illumination light
intensity is a predetermined fraction or percentage of the maximum
intensity of illumination in the area or spot. In the description
of the preferred embodiment, for example, the boundary is where the
light illumination intensity is 1/e.sup.2 of the maximum intensity
of illumination in the area or spot, e being the natural number.
The minimum dimension is the minimum distance between two parallel
lines that enclose between them the boundary of the area or spot.
The term "minimum width" is explained in more detail below.
[0009] Another consideration of the invention is to provide an
adequate throughput while data is collected at a moderate rate for
defect detection so that the data collection and processing system
employed need not be overly complex and expensive.
[0010] Thus another aspect of the invention is directed towards a
method for detecting anamolies on the surface of a semiconductor
wafer, comprising directing a beam of light towards a surface to
illuminate an area of the surface defining a spot having a spot
size whose minimum width is in the range of about 5 to 15 microns,
causing relative motion between the beam and the wafer so that the
beam scans a path covering the entire surface; and collecting light
scattered along said path for detecting anamolies. The spot size
and the directing and causing steps are such that the beam scanning
substantially inspects the entire surface of the wafer at a
throughput in excess of about 40 wafers per hour for 150 millimeter
diameter wafers, at a throughput in excess of about 20 wafers per
hour for 200 millimeter diameter wafers, and at a throughput in
excess of about 10 wafers per hour for 300 millimeter diameter
wafers.
[0011] Yet another aspect of the invention is directed towards a
method for detecting anamolies on a surface, comprising the steps
of directing a beam of light towards the surface to illuminate an
area of the surface defining a spot having a spot size whose
minimum width is in the range of about 5 to 15 microns, causing
relative motion between the beam and the surface so that the beam
scans a path covering substantially the entire surface; and
collecting light scattered along said path for detecting anamolies.
The spot size and said directing and causing steps are such that
the surface is inspected at a speed not less than about 1.5
cm.sup.2/s.
[0012] Still another aspect of the invention is directed towards a
method for detecting anamolies on a surface, comprising the steps
of directing a beam of light towards said surface to illuminate an
area of the surface defining a spot having a spot size whose
minimum width is in the range of about 5 to 15 microns, causing
relative motion between the beam and the surface so that the beam
scans a path covering substantially the entire surface; and
collecting light scattered along said path for detecting anamolies.
The surface has dimensions of not less than 200 millimeters in any
direction along the surface. The directing and causing steps are
such that the beam scans substantially the entire surface in about
50 to 90 seconds.
[0013] Another aspect of the invention is directed towards a system
for detecting anamolies on a surface, comprising means for
directing a beam of light at a grazing angle toward said surface;
means for causing relative motion between the beam and the surface
so that the beam scans a scan path covering substantially the
entire surface; and means for collecting light scattered along said
path for detecting anamolies. The scan path includes a plurality of
arrays of scan path segments, wherein each of at least some of such
scan path segments has a span shorter than the dimensions of the
surface.
[0014] One more aspect of the invention is directed towards a
system for detecting anamolies on a surface of a semiconductor
wafer, comprising means for directing a beam of light towards said
surface to illuminate an area of the surface defining a spot having
a spot size whose minimum width is in the range of about 5 to 15
microns; means for causing relative motion between the beam and the
wafer so that the beam scans a path covering substantially the
entire surface; and means for collecting light scattered along said
path for detecting anamolies. The spot size and said directing and
causing means are such that the beam scanning substantially
inspects the entire surface of the wafer at a throughput in excess
of about 40 wafers per hour for 150 millimeter diameter wafers, at
a throughput in excess of about 20 wafers per hour for 200
millimeter diameter wafers, and at a throughput in excess of about
10 wafers per hour for 300 millimeter diameter wafers.
[0015] Yet another aspect of the invention is directed towards a
system for detecting anamolies on a surface, comprising means for
directing a beam of light toward said surface to illuminate an area
of the surface defining a spot having a spot size whose minimum
width is in the range of about 5 to 15 microns; means for causing
relative motion between the beam and the surface so that the beam
scans a path covering substantially the entire surface; and means
for collecting light scattered along said path for detecting
anamolies. The spot size and said directing and causing means are
such that the surface is inspected at a speed not less than about
1.5 cm.sup.2/s.
[0016] Still one more aspect of the invention is directed towards a
system for detecting anamolies on a surface, comprising means for
directing a beam of light toward said surface to illuminate an area
of the surface defining a spot having a spot size whose minimum
width is in the range of about 5 to 15 microns; means for causing
relative motion between the beam and the surface so that the beam
scans a path covering substantially the entire surface; and means
for collecting light scattered along said path for detecting
anamolies. The surface has dimensions of not less than 200
millimeters in any direction along the surface. The directing and
causing means are such that the beam scans substantially the entire
surface in about 50 to 90 seconds.
[0017] It is a further object of the present invention to classify
detected anomalies and determine their size while increasing the
confidence and accuracy of the detection system by reducing false
counts.
[0018] These objects have been achieved with an apparatus and
method for detecting anomalies of sub-micron size, including
pattern defects and particulate contaminants, on both patterned and
unpatterned wafer surfaces. For the purposes of this application, a
particulate contaminant is defined as foreign material resting on a
surface, generally protruding out of the plane of the surface. A
pattern defect is in the plane of the surface and is usually
induced by contaminants during a photolithographic processing step.
The device employs a plurality of collector channels symmetrically
disposed, in the azimuth, on opposite sides of the center of a scan
line. In addition to the collector channels, other detector
channels are employed to enhance the detection of anomalies. The
collector and detector channels are collectively referred to as
inspection channels. Also, an interchannel communication apparatus
is employed to compare and adjust data received from each of the
inspection channels which facilitate detecting and characterizing
anomalies. A laser beam illuminates a localized spot on a wafer
surface with the beam having a grazing angle of incidence, and the
spot is scanned over a short scan line. The wafer is orientated so
that the streets of the patterns on the die are not oblique with
respect to the scan line, i.e., the streets are either
perpendicular or parallel to the scan line. The surface is moved in
a serpentine fashion, along adjacent striped regions, as the spot
is scanned over its entire area. The position of the inspection
channels, as well as the polarization of the beam, allows
distinguishing, inter alia, pattern defects from particulate
contaminants. The detector channels include an imaging channel
which combines the advantages of a scanning system and an imaging
system while improving signal/background ratio of the present
system. The inspection channels collect light and feed it to a
light detector for producing an electrical signal corresponding to
the collected light intensity. The interchannel communication
apparatus is a processor which stores, in memory, the information
carried by the signals from the inspection channels, with the
memory addresses corresponding to spatial positions on the surface.
The processor constructs maps from the stored information,
representing the anomalies detected on the surface. The maps from
the inspection channels are compared by performing various
algorithms and logical operations, e.g., OR, AND and XOR, to
characterize the detected anomalies.
[0019] In operation, each wafer is scanned with a beam incident
thereon at a grazing angle and the light scattered and specularly
reflected from the wafer's surface are simultaneously collected
with the above mentioned inspection channels. Previously, the wafer
has been aligned so that the streets on the die are not oblique
with respect to the scan line. Light collected is converted into
electrical signals which are further processed by dedicated
electronics. A processor analyzes the information carried by the
signals and produces various maps representing the light intensity
detected at various beam positions. The maps are compared either in
the analog domain or digitally to identify and characterize
anomalies. If compared digitally, the maps are binarized which
allows performing various algorithmic and logical, e.g. OR, AND and
XOR, operations on the data they represent, thereby allowing a user
to choose a desired level of confidence in the detected anomalies.
The binarization can take place against either a constant or a
variable threshold, further reducing the occurrence of false
counts. The variable threshold is dependent upon the local
reflectivity and can be derived from a reflectivity channel which
determines local reflectivity of the surface based upon detecting
specularly reflected light.
[0020] The invention has advantages over the previous scanning
techniques in that it provides a small spot that scans at speeds
far in excess of those of the prior art, while providing the added
feature of classifying anomalies. Further, controlling the
polarization of the incident beam and the light detected results in
an excellent ratio of particle to pattern signal.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1A is a schematic view of an elliptical-shaped
illuminated area or spot on a surface to be inspected to illustrate
the invention.
[0022] FIG. 1B is a graphical illustration of the illumination
intensity across the width or short axis of the elliptical spot of
FIG. 1A for defining a boundary of the spot and to illustrate the
invention.
[0023] FIG. 2 show partially in perspective and partially in block
diagram form a system for inspecting anamolies of a semiconductor
wafer surface to illustrate the preferred embodiment of the
invention.
[0024] FIG. 3 is a perspective view showing in more detail the
illumination and collection features of the system of FIG. 2.
[0025] FIG. 4 is a schematic view of a small portion of a
semiconductor wafer surface illustrating the scan path of an
illumination spot on the surface to illustrate the preferred
embodiment.
[0026] FIG. 5 is a schematic view illustrating the illumination and
collection angles of the system of FIG. 3.
[0027] FIG. 6 is a schematic view of three elliptical illuminated
areas or spots to illustrate the scanning and data acquisition
processes of this invention.
[0028] FIGS. 7A, 7B are side views illustrating two different
polarization schemes employed by present invention for illuminating
a surface to be inspected.
[0029] FIG. 8 is a simplified perspective plan view of the
illumination and collection optics of the present invention.
[0030] FIG. 9 is a top view of the illumination and collection
optics shown in FIG. 1.
[0031] FIG. 10 is a detailed view showing the scan path of a spot
on a wafer surface.
[0032] FIG. 11 is a detailed view of a collection channel shown in
FIG. 1.
[0033] FIGS. 12A, 12B is a plan view showing a polarization scheme
employed by the present invention.
[0034] FIG. 13 is a graph of an electrical signal amplitude (I)
versus beam scan position (X) on a wafer produced by the method of
the present invention using the apparatus shown in FIG. 8.
[0035] FIGS. 14A-14E is a top view of a display derived from a scan
of the wafer, as shown in FIG. 10.
[0036] FIG. 15 is a plan view of an imaging channel shown in FIG.
8.
[0037] For simplicity, identical components in the different
figures of this invention are labeled by the same numerals.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0038] FIG. 1A is a schematic view of an elliptical-shaped
illuminated area (or spot) of a surface inspected by the system of
this invention to illustrate the invention. As explained below, the
laser beam illuminating the surface inspected approach the surface
at a grazing angle, so that even though the illumination beam has a
generally circular cross-section, the area illuminated is
elliptical in shape such as area 10 in FIG. 1A. As known to those
skilled in the art, in light beams such as laser beams, the
intensity of the light typically does not have a flat distribution
and does not fall off abruptly to zero across the boundary of the
spot illuminated, such as at boundary 10a of spot 10 of FIG. 1A.
Instead, the intensity falls off at the outer edge of the
illuminated spot at a certain inclined slope, so that instead of
sharp boundaries such as boundary 10a illustrated in FIG. 1A, the
boundary is typically blurred and forms a band of decreasing
intensity at increasing distance away from the center of the
illuminated area.
[0039] In many lasers, the laser beam produced has a Gaussian
intensity distribution, such as that shown in FIG. 1B. FIG. 1B is a
graphical illustration of the spatial distribution of the
illumination intensity in the Y direction of a laser beam that is
used in the preferred embodiment to illuminate spot 10 of a surface
to be inspected as shown in FIG. 1A, and thus is also the
illumination intensity distribution across spot 10 in the Y
direction. As shown in FIG. 1B, the illumination intensity has been
normalized so that the peak intensity is 1, and the illumination
intensity has a Gaussian distribution in the Y direction as well as
in the X direction. Points 12 and 14 are at spatial locations y1
and y5 at which points the illumination intensity drops to 1/e of
the peak intensity, where e is the natural number. AS used herein
to describe the preferred embodiment, the minimum width of spot 10
is the distance between these two points (distance between y1 and
y5) is the length of the short axis of elliptical illuminated area
10 and denoted as the width w in FIG. 1A. The spot 10 is defined by
the area within a boundary 10a where the illumination is 1/e.sup.2
of that of the maximum intensity of illumination at the center of
the spot.
[0040] As a broader definition, "minimum width" of the elliptical
spot 10a may be defined as the minimum distance between two
parallel lines that enclose between them the boundary of the area
or spot. In reference to spot 10 in FIG. 1A, for example, if one
were to draw two parallel lines enclosing the boundary 10a, such as
lines q1, q2, the distance between lines q1, q2 is d, which is
minimized when both q1, q2 touch the boundary 10a. The distance d
is minimum for all directions of q1, q2 when the lines q1, q2
coincide with grid lines y1, y5, so that the minimum width of the
spot 10 is w. Even where 10a is not an ellipse, but is of another
shape such as rectangular, square, or irregular in shape, the same
broader definition is applicable.
[0041] FIG. 1B shows only the main lobe of the laser or light beam.
It is known that the main lobe is also accompanied by sidelobes, so
that areas of the surface outside of area or spot 10 would also be
illuminated; scattering by structures of the surface of the light
in the sidelobes and collected by the detectors causes noise.
[0042] In the description above, it is indicated that for a spot
which is relatively small compared to the size of the surface to be
inspected, it will be difficult to maintain uniformity of the spot
across a scan line which spans the entire length or width of the
wafer. In reference to FIGS. 1A, 1B, variation in the minimum width
(as defined above) w of the main lobe of the focal plane intensity
distribution, and the level of the sidelobes is a measure of the
uniformity of the spot 10 as the beam scans across the surface.
Where the minimum width and the sidelobes level vary little over
the scan line, the spot is said to be uniform. In other words, when
the spot size is relatively small compared to the size of the
surface to be inspected, it will be difficult to maintain uniform
width of the main lobe and uniform level of the sidelobes of the
focal plane intensity distribution throughout the extent of a long
scan line across the entire width of the wafer. A variation in
either one of these two parameters (width of main lobe and sidelobe
level) leads to a variation in detection sensitivity along the scan
direction which is undesirable.
[0043] In view of the above problems, Applicants have invented a
surface inspection system where the size of the area illuminated by
the scanning light beam can be reduced while maintaining uniform
detection sensitivity by causing the scanning light beam to scan
short scan path segments having a spatial span less than the
dimension of the surface it is scanning, as illustrated in the
preferred embodiment in FIGS. 2 and 4, where these short scan path
segments are not connected together but are located so that they
form arrays of scan path segments as illustrated in more detail
below.
[0044] The surface inspection system of this invention will now be
described in reference to FIGS. 2 and 3. As shown in FIG. 2, system
20 includes a laser 22 providing a laser beam 24. Beam 24 is
expanded by beam expander 26 and the expanded beam 28 is deflected
by acousto-optic deflector (AOD) 30 into a defected beam 32. The
deflected beam 32 is passed through post-AOD and polarization
selection optics 34 and the resulting beam is focused by
telecentric scan lens 36 onto a spot 10 on surface 40 to be
inspected, such as that of a semiconductor wafer, photomask or
ceramic tile, patterned or unpatterned.
[0045] In order to move the illuminated area that is focused onto
surface 40 for scanning the entire surface, the AOD 30 causes the
deflected beam 32 to change in direction, thereby causing the
illuminated spot 10 on surface 40 to be scanned along a scan line
50. As shown in FIG. 2, scan line 50 is preferably a straight line
having a length which is smaller than the dimension of surface 40
along the same direction as the scan line. Even where line 50 is
curved, its span is less than the dimension of surface 40 along the
same general direction. After the illuminated spot has completed
scanning surface 40 along scan line 50, surface 40 of the wafer is
moved along the X axis so that the illuminated area of the surface
moves along arrow 52 and AOD 30 causes the illuminated spot to scan
along a scan line parallel to scan line 50 and in adjacent position
spaced apart from scan line 50 along the negative X axis. After the
illuminated spot has covered such scan line, surface 40 is moved by
a small distance so that the area of the surface to be illuminated
is moved along direction 52 in order to scan an adjacent scan line
at a different X position. As described below, this small distance
preferably is equal to about one quarter of the height of spot 10.
This process is repeated until the illuminated spot has covered
strip 54; at this point in time the illuminated area is at or close
to the edge 54a. At such point, the surface 40 is moved along the Y
direction by about the length of scan line 50 in order to scan and
cover an adjacent strip 56, beginning at a position at or close to
edge 56a. The surface in strip 56 is then covered by short scan
lines such as 50 in a similar manner until the other end or edge
56b of strip 56 is reached at which point surface 50 is again moved
along the Y direction for scanning strip 58. This process is
repeated prior to the scanning of strip 54, 56, 58 and continues
after the scanning of such strips until the entire surface 40 is
scanned. Surface 40 is therefore scanned by scanning a plurality of
arrays of short path segments the totality of which would cover
substantially the entire surface 40.
[0046] FIG. 4 is an exploded view of a portion of the two strips
54, 56 and smaller portions of two other neighboring strips to
illustrate in more detail the above-described scanning process. In
the preferred embodiment as shown in FIG. 4, the optical beam 38
scans in only one direction as illustrated by the arrows of scan
path segments 50, 50', 50", 50'". Scan path 50 has an effective
start location at 72 and spot 10 moves to the right therefrom until
it reaches the border 55 between strips 54 and 56. Upon reaching
border 55, a stage (see FIG. 3) moves the surface 40 in the X
direction perpendicular to the scanning direction Y and the spot
assumes the new start position 74 and moves along a scan line 50'
parallel to scan line 50. The movement of the spot 10 along scan
lines 50, 50', 50", 50'" and so on is achieved by means of AOD 30
as explained below.
[0047] The deflection of beam 32 by AOD 30 is controlled by chirp
generator 80 which generates a chirp signal. The chirp signal is
amplified by amplifier 82 and applied to the transducer portion of
AOD 30 for generating sound waves to cause deflection of beam 32 in
a manner known to those skilled in the art. For a detailed
description of the operation of the AOD, see "Acoustooptic Scanners
and Modulators," by Milton Gottlieb in Optical Scanning, ed. by
Gerald F. Marshall, Dekker 1991, pp. 615-685. Briefly, the sound
waves generated by the transducer portion of AOD 30 modulates the
optical refractive index of an acoustooptic crystal in a periodic
fashion thereby leading to deflection of beam 32. Chirp generator
80 generates appropriate signals so that after being focused by
lens 36, the deflection of beam 32 causes the focused beam to scan
along a scan line such as line 50 in the manner described.
[0048] Chirp generator 80 is controlled by timing electronic
circuit 84 which in the preferred embodiment includes a
microprocessor. The microprocessor supplies the beginning and end
frequencies f1, f2 to the chirp generator 80 for generating
appropriate chirp signals to cause the deflection of beam 32 within
a predetermined range of deflection angles determined by the
frequencies f1, f2. The auto-position sensor (APS) optics 90 and
APS electronics 92 are used to detect the level or height of
surface 40 and form a part of the Related Application. Detectors
such as detector 111b collects light scattered by anamolies as well
as the surface and other structures thereon along scan line 50 and
provides output signals to a processor in order to detect and
analyze the characteristics of the anamolies.
[0049] FIG. 3 is a perspective view of system 20 of FIG. 2 showing
in more detail the arrangement of the collection or detection
channels to illustrated the preferred embodiment. As shown in FIG.
3, four collection channels are used, two channels 110a, 110b for
collecting scattered light that is within the respective ranges of
azimuthal angles of -(75-105).degree. and (75-105).degree.. Two
additional collection channels 111a, 111b are also employed for
detecting forward scattered light that is within the respective
ranges of azimuthal angles of -(30-60).degree. and (30-60).degree..
If desired, it is of course possible to employ four independent
collection channels with other different solid angles of
collection, two of said collection channels located in the forward
direction to collect light in the forward direction centered
substantially at .+-.45.degree. azimuthally and two of the channels
are located to collect light centered substantially at
.+-.90.degree. azimuthally.
[0050] FIG. 5 is a top view of the angles of collection of the four
detectors. As shown in FIG. 5, the solid angles of collection of
channels 110a, 100b are labeled .PHI..sub.1 and those for channels
111a, 111b are labeled .PHI..sub.2. To simplify the drawing, the
components between laser 22 and focus beam 38 are not shown in FIG.
5. In reference to FIG. 3, system 20 also includes an imaging
channel 121 and an alignment/registration channel 122.
[0051] Surface 40 may be smooth (118) or patterned (119). The
incident focus beam 38 is preferably in the range of about
10-85.degree. to the normal direction 150 to the surface 40 and
more preferably within the range of 50-80.degree. from the normal;
in FIG. 3, this angle is labelled .theta.. The four channels of
collection are preferably at elevation angles .alpha. that will
collect scattered light from 3-30.degree. from the plane of surface
40.
[0052] Improved Sensitivity of Detection
[0053] From the point of view of sensitivity of detection, it is
desirable to design the illumination optics portion of system 20 so
that the minimum width w of the illuminated spot 10 is minimized.
The minimum width w is proportional to the focal length of lens 36
and inversely proportional to the beam diameter of beam 28 and 32.
Therefore, the minimum width w can be reduced by reducing the focal
length of lens 36 or increasing diameter of beam 28, or both. If
the focal length of lens 36 is increased, however, this will
increase the length of scan line 50 which may be undesirable. If
the diameter of beam 28 becomes comparable to the clear aperture of
the crystal in AOD 30, this will produce higher level sidelobes
which is undesirable. As noted above, increased level of sidelobes
will increase background signal level. Applicants discovered that
it is preferable for the ratio k between the clear aperture of the
crystal in the AOD 30 to diameter of beam 28 and 32 to exceed
1.2.
[0054] It is possible to increase the beam diameter of beam 28 and
32 by employing a long AOD crystal, while maintaining k to be above
1.2. However, in addition to cost considerations, a larger AOD
crystal will cause larger losses, thereby degrading the diffraction
efficiency of the AOD device. For this reason, it is desirable to
employ AOD crystals that are as small as possible, while at the
same time meeting the sensitivity and throughput requirements.
Assuming that the beam 28 that is entering the AOD 30 has a
Gaussian intensity profile, the clear aperture of the AOD, D,
satisfies"
D=4klv/7.pi.w.DELTA.f, (1)
[0055] where .pi. is the ratio of the circumference of a circle to
its diameter.
[0056] Where l is the scan line of scan path segment 50, v is the
acoustic velocity in the AOD crystal 30, w is the length of the
short axis of the elliptical spot (or the minimum width of the spot
if not elliptical) on surface 40, .DELTA.f or (f2-f1) is the
bandwidth of the AOD 30. The constant k is preferably in the range
1.2-5. In one embodiment, k is 1.7 and l is in the range of about
2-10 millimeters.
[0057] Throughput Considerations
[0058] For a semiconductor wafer inspection instrument to be used
for wafer inspection in actual production for inspecting the entire
surface of the wafer, throughput considerations are paramount.
Therefore, in addition to sensitivity capability described above,
it is also desirable for the wafer inspection system of this
invention to have a high throughput. The time required for
inspecting semiconductor wafers first includes the time required
for the illuminating light beam to scan the entire surface of the
wafer. To perform the above-described short scan path segment
scans, the time required to scan the entire surface depends on a
number of factors. One factor obviously is the angle of
illumination of the illuminating beam, or the value of 0, that is
the angle between the illuminating beam and normal 150 to surface
40 to be inspected shown in FIG. 3. The larger the value of 0 (that
is, the smaller the grazing angle of incidence), the more elongated
would be the shape of the spot 10 in FIG. 1A, and the larger is the
area being inspected. Another factor affecting throughput is the
fact that the intensity distribution of the illuminating beam is
typically not flat but varies, such as in the form of a Gaussian
distribution. Therefore, the intensity of scattering from a
location on a surface would depend on the intensity of the
illuminating light at that location. In order to compensate for
such variation of intensity, a number of data points are obtained
from the scattering from the particular location of the surface as
the spot is moved across the location in a manner illustrated in
FIG. 6 described below.
[0059] FIG. 6 is a schematic view of three positions of the
illuminated area on a surface to be inspected to illustrate the
scanning and data gathering process of system 20. As shown in FIG.
6, at one instant in time, beam 38 illuminates an area 10 on
surface 40. Area or spot 10 is divided into sixteen pixels by grid
lines x1-x5, y1-y5. In this context, the term "pixel" is meaningful
only in reference to the taking of data samples across the
intensity distribution such as that in FIG. 1B and subsequent data
processing and is borrowed from data sampling and processing in
other technologies such as video technology. The pixel that is
bounded by grid lines x2, x3 and y2, y3 is pixel P shown as a
shaded area in FIG. 6. If there is an anamoly in this pixel P, and
if the light illuminating pixel P has the intensity distribution as
shown in FIG. 1B with a high intensity level between grid lines y2
and y3, light scattered by the anamoly would also have a high
intensity. However, as the beam moves along the Y axis so that the
area 10' is illuminated instead, pixel P is still illuminated but
at a lower intensity level of that between grid lines y1 and y2; in
reference to FIG. 1B, the intensity of the illumination is that
between grid lines y1 and y2 in FIG. 1B. Therefore, if the sampling
rate employed by the processor 200 in FIG. 3 for processing light
detected by the collection or collector channels 110a, 110b, 111a,
111b is such that a sample is taken when the illuminating beam is
in position 10 and when the illuminating beam is in position 10',
then two data points will be recorded. Thus if pixel P contains an
anamoly, then two data points will be taken, one when the
illumination is at a higher level as illustrated by data point D2
in FIG. 1B and another one when the illumination is at a lower
level, illustrated at data point D1 in FIG. 1B. If position 10 is
not the starting position in the short scan path segment 50
illustrated in FIGS. 3 and 4, then two prior samples would have
been taken prior to the time when the illuminating beam illuminates
the surface 40 in position 10, so that the processor would have
obtained two more data points D3, D4 corresponding to the prior
positions of the illuminating beam when light of intensity values
between grid lines y3, y4 and between y4, y5 respectively
illuminates such pixel P. In other words, four separate data points
D1-D4 would have been taken of the light scattered by the anamoly
in pixel P as the illumination beam illuminates pixel P when
scanning along the Y direction.
[0060] In most laser beams, the beam intensity has a Gaussian
distribution not only in the Y direction but also in the X
direction. For this reason, after the illuminating beam completes
the scanning operation for scanning a short scan path segment such
as segment 50 as shown in FIG. 4, and when the illuminating beam
returns to position 74 for scanning the adjacent short scan path
segment 50', it is desirable for the illuminated area along path
50' to overlap that of scan path 50 so that multiple samples or
data points can again be taken also along the X direction as well
as along the Y direction. Spot 10 is not drawn to scale in FIG. 4
to show overlap between adjacent scan segments. Therefore, when the
illumination beam is scanning along scan line 50' from starting
position 74 as shown in FIG. 4, the area illuminated would overlap
spot 10; this overlapping spot is 10" as shown in FIG. 6, where the
spot 10" is displaced along the negative X direction relative to
spot 10 by one quarter of the long axis of the ellipse 10 and
10".
[0061] As described above, the minimum width (that is, length of
short axis) of the spots 10, 10', 10" is w. If the angle between
the illuminating light beam and normal 150 to the surface 40 to be
inspected is .theta. as shown in FIG. 3, then the magnitude of the
long axis of the ellipse 10, 10', 10" is w/cos .theta.. Therefore,
in each short scan path segment, the area illuminated sequentially
by the illuminating light beam is given by (w/cos .theta.)*l, where
l is the length of the scan path segment such as 50. Thus if the
radius of surface 40 is R and T is the time it takes for the beam
to scan the short scan path segment, then the time it takes for the
illuminating beam to scan across the entire wafer is given by
N.pi.R.sup.2Tcos.theta./lw (where the duty factor and the time
required for illumination optics to move the beam between strips,
such as strips 54, 56 have not been accounted for). In this
expression, N is the number of pixels along the X axis in each spot
such as 10, 10', 10", since each pixel on the surface will be
illuminated N number of times during the scanning process to
account for the variation of intensity of illumination in the X
direction as described above. In the preferred embodiment
illustrated in FIG. 6, where four data points are taken in both the
X and Y directions, N has the value 4.
[0062] In the scanning process described above in reference to
FIGS. 2-4, it is noted that it will require time for the
illumination optics to move the illumination spot between strips,
such as strips 54 and 56. If .tau. is the time required to move the
illumination spot between strips, then this additional time should
be accounted for to give the total time required to scan the entire
wafer surface. In the preferred embodiment described above, a stage
124 which includes a motor is used in order to move the surface so
as to move the illumination spot from the position for scanning one
strip on the surface to the adjacent strip as shown in FIGS. 2 and
3. For a circular wafer of radius R, the spot will need to be moved
2R/.eta.l times between adjacent strips to move the spot across all
the strips on the entire wafer, so that the additional time
required is 2R.tau./.eta.l, where .eta. is the duty factor
(explained below).
[0063] As known to those skilled in the art, when AOD 30 is used to
cause beam 38 to scan along each short scan path segment such as
50, time will be required at the beginning of the scan for the
sound waves generated by the transducer portion of the AOD to reach
the far end of the AOD crystal so as to begin deflecting the beam.
This is accounted for by a quantity called the duty factor .eta.
given by equation 2 below, and therefore, the total t.sub.s it
takes system 20 to scan the entire surface of a wafer with radius R
is given by equation 3 below: 1 = 1 - 4 kl wT f ( 2 ) t s = N R 2 T
cos + 2 R w lw ( 3 )
[0064] From equation 3 above, it is evident that the shorter the
time T to scan along a scan path segment such as 50, the shorter
will be the time required to scan the entire wafer and therefore
the higher the throughput. The time T is referred to as the chirp
duration which also determines the data rate. The speed of the
electronic circuit for processing the data ultimately sets a lower
limit for the chirp duration.
[0065] From equation 1 above, for a given spot size, length of the
scan path segment and the value of k, it is evident that the larger
the bandwidth .DELTA.f or f2-f1, the smaller will be the clear
aperture required of the AOD. To get maximum bandwidth from the
AOD, the AOD should be operated at the highest possible frequency
and one then expects to get one octave bandwidth around the center
frequency of the transducer. However, the acoustic losses in the
AOD crystal increase with the center frequency of operation. Large
acoustic losses can cause two major problems: reduction in
diffraction efficiency and thermal errors induced in the crystal. A
reduction in the diffraction efficiency reduces the sensitivity of
the system to small particles. When the AOD transducer is operated
at high frequencies, more of the acoustic energy will be converted
into heat which sets up thermal gradients in the AOD crystal. Such
thermal gradients would cause errors by degrading the focal spot
which in turn leads to a reduction in sensitivity for detecting
anamolies. It is therefore advantageous to minimize the acoustic
losses by selecting as low a center frequency of the transducer as
possible. A compromise should then be found to yield acceptable
detection sensitivity as well as acceptable throughput. Applicants
found that a center frequency in the range of 50-300 megahertz and
a bandwidth preferably within the range of 50-250 megahertz would
be acceptable. The AOD 30 is preferably driven by a linear
frequency modulated (FM) chirp signal from generator 80 in FIG. 2.
The quantity .eta.l is the effective length of the scan path
segment; in the preferred embodiment the effective length is in the
range of 2 to 10 mm but more preferably has a value of about 5.47
mm, where l has the value of 6.2 mm.
[0066] From equation 3 above, it is seen that the larger the angle
.theta., the higher will be the throughput, since the illuminated
spot will cover a larger area of the surface. But as noted above,
the larger the spot size, the lower will be the sensitivity of
detection. In the preferred embodiment, .theta. is in the range of
10-85.degree. and more preferably in the range of
50-80.degree..
[0067] Also from equation 3 above, it is evident that the larger
the number of samples taken across the illuminated spot diameter,
the more time it would take to scan the wafer. In the preferred
embodiment, the number of samples taken across the illuminated spot
diameter along both orthogonal axes (X, Y) is in the range of 2-10.
Where four samples are taken along at least the X axis, N is 4 in
equation 3.
[0068] For sensitivity considerations, it is preferable for the
minimum width w of the illuminated area to be in the range of 5-15
microns. If .theta. is in the range of 50-80.degree., then the
illuminating beam will illuminate the scan path segments such as 50
at such speed that the surface is inspected at a speed not less
than about 2.5 cm.sup.2/s, and more preferably in a range of about
2.5-3.8 cm.sup.2/s.
[0069] From equation 3 above, if the time required for moving the
wafer or the illumination beam so that the illuminated spot is
transferred between adjacent strips such as strips 54, 56 is taken
into account, then the average speed for scanning the entire
surface 40 will be reduced compared to that for scanning a short
scan path segment such as segment 50. Furthermore, the speed for
inspecting the entire wafer is further reduced because each pixel
on the wafer is scanned multiple times as described above in
reference to FIG. 6. If the value of T is about 0.3 seconds, and
where the scan speed along each scan path segment is not less than
2.5 cm.sup.2/s, then the average speed for the illumination beam
scanning the entire surface is not less than about 1.5 cm.sup.2/s.
In the preferred embodiment, the average speed is preferably within
the range of about 1.5-5 cm.sup.2/s. If the surface 40 scanned has
dimensions of not less than 200 millimeters in any direction along
the surface, then the illumination beam will scan the entire
surface in about 50-90 seconds. As noted above, the length of the
scan path segments such as segment 50 is preferably small compared
to the dimensions of the surface 40 inspected. In the preferred
embodiment, these segments are substantially in the range of about
2-10 millimeters.
[0070] In the preferred embodiment, generator 80 supplies a linear
FM chirp signal to drive the AOD so that the chirp duration is
preferably in the range of 20-200 microseconds, and more preferably
in the range of about 80-120 microseconds. The beam 28 before
deflection by the AOD 30 has at least one cross-sectional dimension
(e.g. the longer dimension) in the range of about 4-12 millimeters.
Preferably, the scan lens 36 is placed substantially at one focal
length away from AOD 30 so that beam 38 scans the surface 40
telecentrically.
[0071] From the above, it will be evident that the objective of the
invention of the high sensitivity and high throughput surface
inspection system has been achieved while moderate data rate (e.g.
22 Mhz) at modest cost for the data sampling and processing
electronics can still be achieved. This system is capable of
inspecting patterned wafers with 0.35 micron design rule, such as
patterned wafers for 64 and 256 megabit DRAM technology. The system
is capable of detecting contaminant particles and pattern defects
on memory and logic devices. With the present state-of-the-art
robotic implementation for removing and replacing wafer 40 on stage
124 ready for system 20 to inspect and the inherent delay (about 25
seconds/wafer) involved therein, system 20 described above is
capable of inspecting in excess of about 40 wafers per hour for 150
millimeter diameter wafers (6-inch wafers), in excess of about 20
wafers per hour for 200 millimeter diameter wafers (8-inch wafers)
and in excess of about 10 wafers per hour for 300 millimeter
diameter wafers (12-inch wafers).
[0072] FIGS. 7A, 7B are side views showing the polarization schemes
employed by the present invention. It is found that by employing
certain polarization schemes, the signal-to-background of the
system can be substantially improved. The polarization scheme
employed may be surface-dependent and may also be used to determine
the composition of the anamoly (such as metallic as opposed to
dielectric material). For pattern defects, the polarization
elements included in the post-AOD and polarization selection optics
34 of FIG. 2 faces the illumination beam in a state of either P or
S polarization. FIG. 7A illustrates the situation where the
illumination beam 214 is in a state of S polarization where the
electrical field E is perpendicular to the plane of incidence which
is defined by the incident beam 214 and the specularly reflected
beam 214a; this plane of incidence is parallel to the plane of the
paper. A vector representation of the beam is shown by a k vector
representing the direction of propagation. The magnetic field is
shown as the H vector. The electric field vector is shown as being
perpendicular to the plane of incidence by representing it with a
dot and labeled E. In FIG. 7B, the beam 214 is in a state of P
polarization where the electric field E is in the plane of
incidence and the plane of the paper. In FIG. 7B the beam 214 is
shown in vector form with a propagation vector k, a magnetic field
vector shown as a dot labeled H where the electric field vector E
is perpendicular to the propagation vector k. Instead of P or S
polarization states, the illumination beam can also have a left or
righthanded circular polarization. Where the polarization state of
the illumination beam is chosen to optimize signal-to-noise
background and for defect detection, the collector or collection
channels include polarization filters to pass light of
predetermined polarization states to enhance detection capability
and signal-to-noise ratio.
[0073] While in the invention described above, the scan path
segments are described and illustrated as straight lines, it will
be understood that it is also possible for curved scan lines to be
employed, such as where the wafer is rotated about an axis instead
of translated along straight lines in the X and Y directions as
described above. While in the preferred embodiment described above,
the short scan path segments form arrays, each array covering a
substantially rectangular strip of the wafer, it will be understood
that other different arrangements of the scan path segments are
possible for covering the entire or substantially the entire
surface 40; such and other variations are within the scope of the
invention. As the spot 10 approaches the edge of surface 40, the
length of the scan path segment may be reduced so that the spot
does not fall outside surface 40. All the advantages described are
obtained even though the segments are of different lengths if each
of at least some of the segments has a span shorter than the
dimensions of the surface. Also, the AOD 30 may be replaced by a
polygonal scanner or galvanometer. While the invention has been
described by reference to preferred embodiments, it will be
understood that modifications and changes can be made without
departing from the scope of the invention which is to be defined
only by the appended claims.
[0074] The present invention, as shown in FIG. 8 is based on the
discovery that the scattering cross section of an anomaly on a
patterned surface is asymmetrical. This in part is due to the
asymmetry of the anomaly itself, or, in the case of particulate
contaminants, the pattern on which a particulate rests changing the
effective scattering cross section of the particle. Taking
advantage of this discovery, a plurality of detectors are provided
that includes groups of collector channels symmetrically disposed
about the circumference of the surface. Although a greater number
of collector channels may be employed in each group, the preferred
embodiment uses two groups of two collector channels, 1010a-b and
1011a-b, disposed symmetrically about the wafer surface 1012 so
that each collector channel within a pair is located at the same
azimuthal angle on opposite sides of the scan line, indicated by
the line B. With collector channels positioned symmetrically in the
azimuth, a substantial reduction in false counts can be obtained.
For example, an anomaly having a symmetrical scattering cross
section, will cause scattered light to impinge on a pair of
collector channels, disposed symmetrically in the azimuth, with the
same intensity. Anomalies with an asymmetrical scattering cross
section will impinge on the same pair of collector channels with
varying intensities. By comparing data representing the intensity
of light impinging on symmetrically disposed collector channels,
signals which are in common, such a pattern signals, may be
discarded. This provides a high confidence level that the resulting
signals are in fact anomalies, and not due to random scattering by
surface features. The data from the channels is compared by
performing various algorithms and logical operations, e.g., OR, AND
and XOR. In addition, examining the data concerning the anomalies
having unidentical signals in the two channels allows determining
the shape and/or composition of them.
[0075] As shown in FIG. 8, a light source 1013, typically a laser,
emits a beam 1014. Beam 1014 is directed towards the pre-deflector
optics 1015, which consists of a half wave-plate, a spatial filter
and several cylindrical lenses, in order to produce an elliptical
beam with a desired polarization that is compatible with the
scanner 1016. The pre-deflector optics 1015 expands the beam 1014
to obtain the appropriate numerical aperture. The post-deflector
optics 1017 includes several cylindrical lenses and an air slit.
Finally, the beam 1014 is brought into focus on the a wafer surface
1012 and scanned along the direction, in the plane of the wafer
surface 1012, indicated by B, perpendicular to the optical axis of
the beam 1014. The type of deflector employed in the apparatus is
application dependent and may include a polygonal mirror or
galvonmeter. However, in the preferred embodiment, deflector 1016
is an Acousto-optic Deflector. The wafer surface 1012 may be smooth
1018 or patterned 1019. In addition to the collector channels
1010a-b and 1011a-b, described above, detector channels are
provided which include a reflectivity/autoposition channel 1020, an
imaging channel 1021 and an alignment/registration channel 1022,
each of which are discussed more fully below.
[0076] The beam 1014 has a wavelength of 488 nm and is produced by
an Argon ion laser. The optical axis 1048 of the beam 1014 is
directed onto the wafer surface 1016 at an angle, .THETA.. This
angle, .THETA., is in the range of 55-85.degree. with respect to
the normal to the wafer surface 1012, depending on the application.
The scanning means includes the deflector 1016 and the translation
stage 1024 upon which the wafer rests. The position of the wafer on
the stage 1024 is maintained in any convenient manner, e.g., vacuum
suction. The stage 1024 moves to partition the surface 1012 into
striped regions, shown as 1025, 1026 and 1027 with the deflector
1016 moving the beam across the width of the striped regions.
[0077] Referring to FIG. 9, the grazing angle of the beam 1014
produces an elliptical spot 1023 on the wafer surface 1012, having
a major axis perpendicular to the scan line. The deflector 1016
scans the spot 1023 across a short scan line equal in length to the
width of striped region 1025 to produce specularly reflected and
scattered light. The spot 1023 is scanned in the direction
indicated, as the stage 1024 moves the wafer perpendicular to the
scan line. This results in the spot 1023 moving within the striped
region 1025, as shown in FIG. 10. The preferred embodiment scans in
only one direction as indicated by scan path 1028. Scan path 1028
has an effective start location at 1029 and the spot 1022 moves to
the right therefrom until is reaches the border 1031 of striped
region 1025. Upon reaching border 1031, the spot 1023 the stage
1024 moves perpendicular to the scan direction and the spot assumes
a new start position 1030 and moves parallel to scan line 1028,
along scan line 1032. The deflector 1016 continues to scan the spot
1023 in this fashion along the entire length of striped region
1025. Upon completion of the scan of striped region 1025, the stage
1024 moves the wafer to permit the scanning of the adjacent striped
region 1026. The effective start location 1033 is positioned so
that the stage 1024 shall move perpendicular to each scan line in a
direction opposite to that when scanning striped region 1024,
thereby forming a serpentine scan. This is demonstrated by scan
paths 1034 and 1035. Moving the stage 1024 to scan adjacent striped
regions in opposite directions substantially reduces the amount of
mechanical movement of the stage while increasing the number of
wafers scanned per hour.
[0078] Referring to FIGS. 8 and 10, light scattered from the wafer
surface 1012 is detected by a plurality of detectors, including
collector channels 1010a-b and 1011a-b. An important aspect of the
collector channels is that they collect light over a fixed solid
angle, dependent upon, inter alia, the elevational and azimuthal
angle of the channel. The optical axis of each collection channel
is positioned at an angle of elevation .psi. in the range of 70-90
degrees, with respect to the normal to the surface 1012. As
discussed above collector channels 1010a and 1010b are
symmetrically positioned at the same azimuthal angle with respect
to beam 1014, on opposite sides of the scan line. Collector
channels 1010a and 1010b are positioned, with respect to the beam
1014, at an azimuthal angle .PHI..sub.1 in the range of about 75 to
about 100 degrees to collect laterally scattered light. Laterally
scattered light is defined as light scattered at azimuthal angles
in the range of about 75 to about 100 degrees, with respect to beam
1014. Similar to collector channels 1010a and 1010b, channels 1011a
and 1011b are positioned on opposite sides of the scan line at the
same azimuthal angle; however, the azimuthal angles .PHI..sub.2 of
channels 1011a and 1011b are in the range of 30 to 60 degrees, to
collect forwardly scattered light. Forwardly scattered light is
defined as light scattered at azimuthal angles in the range of 30
to 60 degrees.
[0079] Providing the groups of collector channels, at differing
azimuthal angles, facilitates classifying detected anomalies, by
taking advantage of a discovery that laterally scattered light is
more sensitive to detecting pattern defects, and forwardly
scattered light is more sensitive to detecting particulate
contaminants. To that end, channels 1010a and 1010b are positioned
to collect laterally scattered light, representing pattern defects,
and channels 1011a and 1011b are provided collect forwardly
scattered light, representing particulate contamination.
[0080] Referring to FIG. 11, each collector channel 1010a-b and
1011a-b includes a lens system 1113 that collects scattered light.
A series of mirrors 1114a-c reflect the light so that it is imaged
onto a photomultiplier tube (PMT) 1115. The PMT 1011 converts the
light impinging thereon into an electrical signal having a voltage
level that is proportional to the light intensity. Positioned at
the Fourier transform plane is a programmable spatial filter 1116
and a variable aperture stop 1117. The programmable spatial filter
1116 allows the system to take advantage of spatial filtering when
periodic features on the surface 1012 are scanned. In addition to
the angle of elevation and the azimuthal angle of each channel, the
variable aperture stop permits varying the elevational collection
angle by limiting the light introduced into the collector channel,
in accordance with the geometry of the features on the wafer
surface 1012. Also located proximate to the Fourier transform plane
is a variable polarization filter 1118. It should be noted, that it
is also possible to place a PMT directly at the Fourier transform
plane.
[0081] Referring to FIGS. 12A and 12B, it was found that by
employing the following polarization schemes, the signal to
background of the system could be substantially improved. To obtain
optimum signal to background, the polarization scheme employed by
the system is surface dependent. It may also be used to determine
the composition of the anomaly, e.g., as composed of metallic or
dielectric material. With respect to pattern defects, the
polarizing element included in the post-scanner optics 1017 will
place the beam 1014 in a state of either P- or S-polarization. A
beam is in a state of S-polarization when its electrical field is
perpendicular to the plane of incidence. The plane of incidence is
parallel to the plane of the paper. It is defined by the surface
1012, beam 1014 and reflected beam 1014b. A vector representation
of the beam is shown by a {right arrow over (k)} vector
representing the direction of propagation. The magnetic field is
shown as the {right arrow over (H)} vector. The electric field
vector is shown as being perpendicular to the plane of incidence by
representing it with a dot {right arrow over (E)}. A beam is in a
state of P-polarization when the electric field is in the plane of
incidence. This is shown in FIG. 12B, where the beam 1014 is shown
in vector form with a propagation vector {right arrow over (k)}, a
magnetic field vector shown as a dot {right arrow over (H)} and the
electric field vector {right arrow over (E)}, perpendicular to the
propagation vector {right arrow over (k)}. Referring also to FIG.
11, if beam 1014 is incident on the surface 1012 in an S state of
polarization, the variable polarization filter 1118 would allow
scattered light in an S state of polarization to pass through it
and attenuate all other scattered light. For example, both
non-polarized or P-polarized light would be attenuated and
S-polarized light would be collected by the collector channels.
Alternatively, optimizing the detection of pattern defects could be
accomplished with an S-polarized beam 1014 and the polarization
filter allowing all scattered light to pass through it. If the beam
1014 is in a P-polarization state, the variable polarization filter
1118 would allow P-polarized light to pass through it and would
attenuate all other scattered light. Alternatively, the
polarization filter could allow all scattered light to be detected
when beam 1014 is P-polarized. This also optimizes detection of
pattern defects. Similarly, if the beam 1014 were incident on the
surface 1012 with either a left or right handed circular
polarization, the collector channels would be very sensitive to
detecting pattern defects by allowing the polarization filter to
pass all the collected light therethrough.
[0082] To detect particulate contaminants on a pattern surface, the
variable polarization filter 1118 would attenuate scattered light
that is not in a P state of polarization, if the beam were
S-polarized. Were beam 1014 in a P state of polarization, the
collector channels would collect scattered light that was
S-polarized, whereby the variable polarization filter 1118 would
attenuate all other scattered light impinging on the channel. For
detecting particulates on a bare surface, beam 1014 would be in a P
state of polarization and the collector channels would collect all
light scattered therefrom to maximize the capture rate.
[0083] Referring to FIG. 13, an electrical signal 1037 is produced
by one of the inspection channels corresponding to an intensity I
of collected scattered light as a beam scans over a scan path. The
abscissa X of the graph in FIG. 13 represents the spatial position
of the beam along the scan path. Signal 1037 is made of a plurality
of discrete samples taken during the scan, e.g., a plurality of
scan lines, each of which were scanned at different positions on a
surface.
[0084] FIGS. 8 and 14A-14E are an example of an interchannel
communication scheme. Shown therein is a resulting display of a map
constructed by a processor 1500 from the signals produced by the
inspection channels. For purposes of this example, FIGS. 14A and
14B represent scattered light detected by a pair of collector
channels. The light detected from the surface consists of a
plurality of signals, shown as spots 1038. These spots may
represent anomalies or false positives: light detected from
features or other non-anomalies present on the surface. The spots
1038 may be stored digitally in the processor memory at addresses
corresponding to spatial positions on the surface. The processor
1500 compares the data stored in memory at addresses represented by
the map shown in FIG. 14A with the data stored in memory
represented by the map shown in FIG. 14B. The data can be compared
by performing various algorithmic or logical operations on it. A
logical OR operation maximizes the capture rate at the expense of a
potential increase in false counts by storing all anomalies
detected between both channels in memory. The composite map shown
in FIG. 14C is the end result of performing a logical OR operation
on the data stored in the processors memory addresses, as
represented by the maps shown in FIGS. 14A and 14B. Alternatively,
a logical AND operation would discard all anomalies that are not
common to both channels, which is the preferred embodiment. The
composite map shown in FIG. 14D is the end result of performing a
logical AND operation on the data stored in the processor's memory
addresses, as represented by the maps shown in FIGS. 14A and 14B.
An exclusive OR operation discards anomalies that are detected on
both channels, keeping only those anomalies which are not commonly
detected, as shown in FIG. 14E. These "suspect" particles would
merit further examination with, inter alia, a high resolution
microscope which could be employed on the system.
[0085] Referring again to FIG. 13, another manner in which to
construct the maps, shown in FIGS. 14A-C, is provided in which only
those positions where the signal 1037 crosses a certain threshold
voltage level are stored in memory, while the remaining signal
portions are discarded. For example, two threshold levels are
shown: a fixed threshold level 1039, and a variable threshold level
1040. At threshold level 1039, peaks 1041-1047 are registered and
stored in memory. At the variable threshold level 1040, as shown,
only peaks 1041, 1043 and 1045 are stored in memory. Using the
threshold voltage level as shown, fewer positions are registered to
form a map thereby making the subsequent processing faster, but at
the risk of failing to detect smaller anomalies. The fixed
threshold level 1039 provides a greater number of positions being
detected, but making the system slower. Typically, the fixed
threshold level 1039 is preset before scanning a wafer, and the
variable threshold level is derived from the
reflectivity/autoposition channel as described below.
[0086] Although the above-described example discussed comparing
maps from signals generated by a pair of collector channels, this
is not the only manner in which the system may operate. It is to be
understood that maps formed from signals generated by the detector
channels may also be compared to identify and classify anomalies,
by performing algorithms and logical operations on the data, as
described above. Comparing signals to a variable threshold level
provides an instructive example, because the threshold level is
derived from the bright field reflectivity/autopositio- n channel
1020, shown in FIG. 8.
[0087] The variable threshold level is dependent upon the local
reflectivity. To that end, the bright field
reflectivity/autoposition channel 1020, is positioned in front of
the beam 1014 to collect specularly reflected light. The bright
field signal derived from this channel carries information
concerning the pattern, local variations in reflectivity and
height. This channel is sensitive to detecting various defects on a
surface. For example, the bright field signal is sensitive to
representing film thickness variations, discoloration, stains and
local changes in dielectric constant. Taking advantage of bright
field signal sensitivity, the bright field signal is used to
produce the variable threshold level 1040, shown in FIG. 13. It is
also used to produce an error height signal, corresponding to a
variation in wafer height, which is fed to a z-stage to adjust the
height accordingly, as well as to normalize the collector and
detection channel signals, whereby the signals from the inspection
channels each are divided by the bright field signal. This removes
the effect of dc signal changes due to surface variations. Finally,
the bright field signal can be used to construct a reflectivity map
of the surface. This channel is basically an unfolded Type I
confocal microscope operating in reflection mode. It is considered
unfolded because the illuminating beam and reflected beams, here,
are not collinear, where as, in a typical reflection confocal
microscope the illuminating and reflected beams are collinear.
[0088] Referring to FIG. 15, the imaging channel is shown to
include a lens assembly 1119 that images scattered light onto a
linear array of sensors 1120 having pixels 1121, e.g.,
charge-coupled detectors. The array 1120 is positioned so that the
pixels are normal to the wafer surface 1012 with the lens assembly
1119 collecting upwardly scattered light. The spot 1023 is focused
and scanned in synchronism with the transferring of a charge
contained in each pixel 1121. This enables charging each pixel 1121
independently of the remaining pixels, thereby activating one pixel
1122 at a time with each pixel positioned so as to receive light
scattered from a unique area of the sample surface, illuminated by
the spot along the scan line. In this manner each pixel forms an
image on the area illuminated by the spot, wherein there is a
one-to-one correlation between a pixel and the spot position along
the scan line. This increases the sensitivity of the system by
improving the signal to background ratio. For example, it can be
shown that for a PMT-based channel, the signal to background is
defined as follows:
P.sub.s/P.sub.b=.sigma./A.sub.bh
[0089] where P.sub.s is the optical power scattered by a particle,
P.sub.b is the background optical power, A.sub.b is the area of the
beam on the surface and .sigma. and h are constants. This shows
that the ratio of the scattering cross section to the area of the
beam determines the signal to background ratio.
[0090] With an imaging-based channel, all the scattered power from
an anomaly is imaged onto one array element. The power distributed
in background, however, is imaged over a range of elements,
depending upon the magnification of the system. Assuming a linear
magnification M, at the image plane the background power over an
area is as follows:
M.sup.2A.sub.b
[0091] providing an effective background power per array element
as
P.sub.b=P.sub.ihA.sub.c/M.sup.2A.sub.b
[0092] where A.sub.c is the area of an array element. Therefore,
the signal to background ratio is given by the following:
P.sub.s/P.sub.b=M.sup.2.sigma./A.sub.ch
[0093] This shows that the signal to background ratio is
independent of the spot diameter, providing an improved signal to
background ratio given by:
i=M.sup.2A.sub.b/A.sub.c
[0094] If imaging is not desired, another PMT-based collector
channel similar to the one shown in FIG. 11 may be employed in lieu
of the imaging channel, to collect upwardly scattered light.
[0095] Referring again to FIG. 8, an alignment and registration
channel 1022 is provided. The channel 1021 has the same design as a
basic collection channel 1010a-b and 1011a-b, but it is positioned
in the plane of incidence so that the signal produced from the
patterns or features on the wafer's surface is at a maximum. The
signal obtained is used to properly align the wafer surface 1012 so
that the streets on the features are not oblique with the scan
line. This also reduces the amount of signal collected by the
collector channels, resulting from scattering by patterns.
[0096] In operation, the beam 1014 is scanned over the surface
1012, producing both scattered and specularly reflected light,
which are simultaneously detected. The light scattered laterally,
forwardly and upwardly is simultaneously detected by the collector
channels and the imaging system. The specularly reflected light
from the wafer's surface 1012 is detected by the bright field
reflectivity/autoposition channel 1020. Light detected by the
inspection channels is converted into electrical signals which are
further processed by dedicated electronics, including a processor
1500. The processor 1500 constructs maps from the signals produced
by the inspection channels. When a plurality of identical dies are
present on the wafer surface 1012, a detection method may be
employed whereby periodic feature comparisons are made between
adjacent die. The processor compares the maps from the inspection
channels either in the analog domain or digitally, by performing
logical operations on the data, e.g., AND, OR and XOR, to detect
anomalies. The processor forms composite maps, each representing
the detected anomalies by a single group of symmetrically disposed
collector channels. The composite maps are then compared so that
the processor may classify the anomalies as either a pattern defect
or particulate contamination. Typically, the wafer surface 1012 has
been aligned so that the streets on the die are not oblique with
respect to the scan line, using the information carried by the
electrical signal produced by the alignment/registration channel.
Proper alignment is a critical feature of this invention, because
periodic feature comparison is performed to locate anomalies.
[0097] While the above described apparatus and method for detecting
anomalies has been described with reference to a wafer surface, it
can easily be seen that anomaly detection is also possible for
photomasks and other surfaces, as well as producing reflectivity
maps of these surfaces. The invention is capable of detecting
anomalies of submicron size and affords the added advantage of
classifying the type of anomaly and identifying its size and
position on the surface. This information is highly useful to wafer
manufacturers as it will permit locating the step in the wafer
manufacturing process at which point an anomaly occurs.
* * * * *