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USOO5479252A
United States Patent [19]
[11] Patent Number:
Worster et al.
[45]
[54] LASER IMAGING SYSTEM FOR
INSPECTION AND ANALYSIS OF
SUB-MICRON PARTICLES
5,479,252
Date of Patent:
Dec. 26, 1995
OTHER PUBLICATIONS
A. F. Slomba et al., “A Laser Flying Spot Scanner for Use
in Automated Fluorescence Antibody Instrumentation,”
Journal of The Association For The Advancement of Medi
cal Instrumentation’ VOL 6, N0 3’ May_Jun_ 1972’ pp_
23O_234_
[75] Inventors: Bruce W. Worster, Saratoga; Dale E.
Crane; Hans J. Hansen, both of
Pleasamon; ChriSmPher R- Farley’
San Jose; Ken K‘ Lee’ Los Altos’ all of
Calif
H. M. Nier, “Automatic Moving Part Measuring Equip
ment,” IBM Technical Disclosure Bulletin, vol. 22, No. 7,
Dec. 1979, pp. 2856-2857.
G. I. Brakenho?" et al., “Confocal Scanning Light Micros
[73] Assignee; Um-apoime Corporation, San Jose,
Calif
copy with High Aperture Immersion Lenses,” Journal of
Microscopy, vol. 117, pt. 2, Nov. 1979, pp. 2l9—232.
(List continued on next page.)
[21] Appl. No.1 80,014
Primary Examiner-Robert P. Limanek
Assistant Examiner—David B. Hardy
[22] Filed:
Attorney, Agent, or Firm-Skjerven, Morrill, MacPherson,
Jun. 17, 1993
[51]
Int. c1.6 ................................................... .. G01N 21/88
Frankhn 8‘ Fuel; Alan H‘ Macpherson
[52]
U.S. Cl. .................. .. 356/237; 356/369; 250559.42;
[57]
[58]
250/559'48
Field Of Search ................................... .. 356/237, 369,
A laser irna in s stem is used to anal ze defects on
g wafers
g y that have been detected
y by patterned
semiconductor
356/317, 445; 250/563, 572
ABSTRACT
wafer defect detecting systems (wafer scanners). The laser
imaging system replaces optical microscope review stations
[56]
'
References Cited
now utilized in the semiconductor fab environment to exam
ine detected optical anomalies that may represent wafer
U‘S‘ PATENT DOCUMENTS
defects. In addition to analyzing defects, the laser imaging
system can perform a variety of microscopic inspection
Re.
Carlsson et
... . ..
. . . . . . ..
functions
including
defect detection
and metrology
The
2,758,502
8/1956 Scott et a1. ................ _. 356/100
et a1‘
"" "
laser imaging System uses confocal laser scanning mime
copy. techniques,_and operates under class I cleanroom
3:049:04?
8/1962 Polanyi 665i.-'::........,............':: 88/14
condm‘ms and wlthout exposure "f the wafers to Operator
contamination or air?ow. Unlike scanning electron micro
scopes (SEMs) that have previously been used for defect
(List continued on next page.)
analysis, the laser imaging system will not damage samples
or slow processing, costs signi?cantly less to implement
than an SEM, can produce a three dimensional image which
provides quantitative dimensional information, and allows
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0052392
0112401
0155247
1185339
2132852
2152697
2184321
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European paL Off ,
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United Kingdom -
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United Kingdom .
United Kingdom .
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WO79/O1027 11/1979
sub-surface viewing of defects lying beneath dielectric lay
ers. The laser imaging system is adaptable to cluster or
in-situ applications, where examination of defects or struc
tures during on-line processing can be performed.
WIPO .
29 Claims, 6 Drawing Sheets
are
r 214
U* E 6
215
CPU Graphics
Processor‘
5,479,252
Page 2
US. PATENT DOCUMENTS
3,187,627
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. . . . ..
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Microscope Images,” International Conference on Elec
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Baer
.. . . .
. ... ... .....
3/1973 Fujiyasu et al.
.. 178/67 R
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Funk et a1. ........ ..
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12/1975 Frosch et a1. ........................... .. 350/17
3/1976 Alien et al. ............................ .. 178/68
9/1976 Browning
.... .. 178/66 R
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8/1977 Bouton et a1. ..
12/1977 Kozma et al. .
4,068,381
340/1463 B
365/125
1/1978 Ballard et a1. ...................... .. 33/1 R
4,125,828
11/1978
4,141,032
2/1979
Resnick et al. .......... ..
Haeusler ...... ..
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7/1979 Christy et a1.
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3/1980 van den Bosch ..
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6/1980 Resnick et al.
4,211,924
7/1980 Miiller et al. ..
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4,236,179
4,255,971
Ruker ........... ..
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3/1981 Dreyfus
Rosencwaig
et a1............................. .. 73/606
4,284,897
8/1981
Sawarnura et al. ............... .. 250/461 B
4,311,358
1/1982
Gibbons et a1.
4,314,763
2/1982 Steigmeier et al.
4,343,993
4,350,892
8/1982 Binnig et al.
9/1982 Kay et al. ........ ..
... .. ..
4,354,114
10/1982 Kamaukhov et al. .
Mirkin
.. ... . . ... .....
. . . . . . ..
350/91
356/237
.... .. 250/306
250/461 2
250/458.1
4,366,380
12/1982
4,379,135
4/1983
Sasaki et a1. ..
.... .. 436/536
4,379,231
4/1983
Shii et a1.
. . . . ..
....... ...
. . . . ..
250/306
250/311
4,405,237
9/1983 Manuccia et al. .
4,406,015
9/1983 Koga ........... ..
4,406,525
9/1983
Itoh et al. .... ..
4,407,008
9/1983
Schmidt et al. ........................ .. 358/93
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256/301
378/50
.. 350/486
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4,485,409
ll/1984
4,549,204
10/1985 Bertero et al. .
4,631,581
Schumaeher .......................... .. 358/294
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Carlsson .................................. .. 358/93
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Nakata et a1.
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117, pt. 2, Nov. 1979, pp. 233-242.
W. Jerry Alford et al., “Laser Scanning Microscopy,” Pro
ceedings of the IEEE, vol. 70, No. 6, Jun. 1982, pp.
641-651.
H. J. B. Marsman et al., “Mechanical Scan System for
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tronic Image Processing, Jul. 26-28, 1982, pp. 101-104.
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. . . . . ..
382/6
10/1992 Ellis ............ ..
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11/1992 Fountain ............................... .. 250/201
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Shura Agadshanyan et al., “Morphoquant-An Automatic
Microimage Analyzer of the JENA Optical Works,” JENA
Review, JR 6, 1977, pp. 270-276.
C. J. R. Sheppard et al., “Optical Microscopy with Extended
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171-186.
N. Aslund et al., “PHOIBOS, A Microscope Scanner
Designed for Micro-Fluorometric Applications, Using
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pp. 338-343.
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demic Press, 1980, pp. 183-225.
P. Davidovits et al., “Scanning Laser Microscope for Bio
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1971, pp. 1615-1619.
I. J. Cox et al., “Scanning Optical Microscope Incorporating
a Digital Framestore and Microcomputer,” 2219 Applied
Optics, vol. 22, May 1983, No. 10, pp. 1474-1478.
Paul Davidovits et al., “Scanning Laser Microscope,”
Nature, vol. 223, Aug. 23, 1969, p. 831.
A. Boyde et al., “Tandem Scanning Reflected Light Micros
copy of Internal Features in Whole Bone and Tooth
Samples,” Journal of Microscopy, vol. 132, Pt. 1, Oct. 1983,
pp. l-7.
Mojmir Petran et a1. “Tandem-Scanning Re?ected Light
Microscope,” Journal of the Optical Society of America, vol.
58, No. 5, May 1968, pp. 661-664.
D. K. Hamilton et al., “Three-Dimensional Surface Mea
surement Using the Confocal Scanning Microscope,”
Applied Physics B 27, 1982, pp. 211-213.
David A. Agard, “Three-Dimensional Architecture of a
Polytene Nucleus,” Nature, vol. 302, Apr. 21, 1983, pp.
C. J. R. Sheppard et al., “Depth of Field in the Scanning
676-680.
Microscope,” Optics Letters, vol. 3, No. 3, Sep. 1978, pp.
Kenneth R. Castleman, Digital Image Processing, 1979
Prentice-Hall, Inc., 1979, pp. 351-359.
115-117.
I. J. Cox et al., “Digital Image Processing of Confocal
Images,” Image And Vision Computing, 1983 Butterworth
IBM Tech. Disclosure Bulletin, vol. 18, No. 12, 1976, p.
1474.
US. Patent
Dec. 26, 1995
Sheet 1 of 6
106
FIG. 1
5,479,252
US. Patent
Dec. 26, 1995
Sheet 2 of 6
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Dec. 26, 1995
Sheet 3 of 6
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Dec. 26, 1995
Sheet 4 of 6
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Dec. 26, 1995
Sheet 5 0f 6
5,479,252
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POSITION
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5,479,252
1
2
LASER IMAGING SYSTEM FOR
INSPECTION AND ANALYSIS OF
SUB-MICRON PARTICLES
The video systems generally cost about three times as
much as the laser scanning systems, i.e., the laser scanning
systems typically cost approximately $350,000 while the
video systems typically cost approximately $1,000,000.
However, while the laser scanning systems are more effec
tive in detecting bumps than in detecting pits, the video
CROSS-REFERENCE TO RELATED
systems work well in detecting either bumps or pits, and can
APPLICATIONS
also sense subsurface defects.
As these wafer scanners were developed, the need to
This application is related to the commonly owned, c0
identify positively the nature, e.g., type of material, type of
pending U.S. patent application entitled “Surface Extraction
defect (defects are classi?ed broadly as particulate or pro
cess ?ow defects; there are many sub-types within each of
from a Three~Dimensional Data Set,” by Ken K. Lee,
application Ser. No. 08/079,193, ?led on the same date as the
these classi?cations), and the precise location and size of the
defects was not appreciated. This information is important
present application and incorporated by reference herein.
15
BACKGROUND OF THE INVENTION
for several reasons. Identi?cation of the nature of the defect
can be used to determine the origin of the defect. The
number, location and size of the defects can be used to
calculate the density of defects in general, and, along with
1. Field of the Invention
identi?cation of the nature of the defects, the density of
This invention relates to lasers and, in particular, to a laser
particular types of defects. This information can then be
imaging system for use in analyzing defects on semicon 20 used to more closely monitor and/or to modify process steps
ductor wafers.
in the chip production process.
2. Related Art
As the need for more precise defect analysis has become
Semiconductor chip manufacturers have increasingly
apparent, semiconductor manufacturers’ demand for the
sought to improve yields in their production processes. Key
ability to “revisit” defects (or a subset of them) found by the
to this effort is the reduction of particulate contamination 25 above-described wafer scanners, for purposes of positive
during wafer processing. As the line widths of features on
identi?cation of the nature, location and size of the defects,
the chip have shrunk from 10 microns several years ago to
has led to the hasty design and production of review stations
one micron and below today (with line widths approaching
based on laboratory microscopes with precision wafer han
0.3 micron or less expected in the next few years), the ability
dling stages that allow an operator to close in on and
to detect and control smaller and smaller particles to achieve
evaluate the previously detected defects. Revisiting of the
higher degrees of cleanliness has become paramount. Addi
defects by the review stations is done off~line from the defect
tionally, production of acceptable chips requires accurate
detection process so as not to limit the throughput of the
performance of each of the process steps carried out on the
wafer scanners. Little engineering was done in the design of
wafer. The value of product on each wafer has also increased
these review stations: in particular with respect to the optics
dramatically, due to the increasing complexity of semicon
and cleanliness (e.g., the review stations typically use off
ductor devices (many more layers and process steps) and the
the-shelf, visible light, research~style microscopes).
development of larger wafers (up to 200 mm diameter),
As noted above, the decreasing line widths of features on
?irther accentuating the need for defect detection and con—
current and future semiconductor chips increase the impor
trol.
40 tance of detection of contaminants and other defects having
Instrument suppliers have addressed a portion of this
a diameter, width, or other characteristic dimension on the
problem by developing defect detecting systems which scan
order of 0.1 to 0.3 microns. The visible light, off-the-shelf
wafers (wafer scanners) during production for anomalous
microscopes currently being used in defect review stations
optical sites that are characteristic of particulate contamina
lack sufficient resolution to resolve defects of such small
size, or to resolve this size structure on larger defects to aid
tion (but may represent other ?aws as well). Defects can be
either a pit or a bump in the surface of the wafer.
In one type of wafer scanner, in which a laser beam is
focussed on and scanned over the surface of the chip (laser
in identi?cation. Visible light scanning microscopes (both
scanning system), anomalous optical sites are identi?ed by
comparing the light scatter from locations on known good
chips to the light scatter from the corresponding locations on
the chips being tested. If the two light scatters are different,
than an anomalous optical site has been detected. Wafer
scanners of this type are made by Tencor Instruments of
Mountain View, Calif. as Model 7500, and by Inspex of
Billerica, Mass. as Model TPC 8500.
In another type of wafer scanner, a video picture is taken
50
the use of conventional microscopes increases the risk of
contamination of the semiconductor chips during the review
process, since a (relatively dirty) human is in close proxim
ity to the wafer surface and because the presence of the
55
Consequently, the semiconductor processing industry has
good chip and compared to a corresponding video picture
taken of a chip to be tested. Typically, these video systems
60
use white light imaging. The video pictures are analyzed by
comparing them on a pixel by pixel basis, i.e., numerical
data representing the video image at each pixel is compared
and, if the difference falls outside of a pre-established
acceptable difference, an anomalous optical site is identi?ed.
65
as, for example, Model 2131.
microscope causes turbulent ?ow near the wafer which tends
to pull in nearby contaminants to the wafer.
with a conventional video camera of the surface of a known
KLA of San Jose, Calif. makes a wafer scanner of this type
white light and laser-based) that are built by modifying
off-the-shelf microscopes can improve the resolution sig
ni?cantly, but they are currently in limited use, mostly as
part of complex and expensive research setups. Additionally,
attempted to use scanning electron microscopes (SEMs) that
will provide increased resolution and perform energy dis
persive (EDX) analysis. In EDX analysis, X-rays are
directed toward the surface of the semiconductor chip. By
measuring the wavelength spectrum of the reflected light,
information can be gleaned regarding the types of material
present on the wafer surface. Unfortunately, EDX analysis
requires high voltage (up to approximately 40,000 volts)
SEMs; bombardment of the wafer surface with electrons
from high voltage SEMs causes damage to the wafer,
5,479,252
3
4
rendering the wafers unusable for further processing.
Recently, low voltage SEMs (100—1000 volts) have seen
microscopy techniques, including multiline visible light
limited use in wafer fabs for “critical dimension” measure
ments of line widths, but low voltage SEMs are too slow to
improving resolution even further due to the shorter wave
use except on a sample basis, and, in addition, provide no
lengths of the ultraviolet light. The laser imaging system has
The laser imaging system utilizes confocal laser scanning
lasers, and can be optionally ?tted with an ultraviolet laser,
analytical (i.e., EDX) capability. Further, in both high and
resolution on the order of 0.1 to 0.2 microns. The laser
low voltage SEMs, the time to load samples into the SEM
and pump down the load-lock chamber containing the SEM
imaging system can also be used for metrology.
Additional capabilities of the instrument include ?uores~
cence of contaminants (for assistance both in locating them
is relatively long, undesirably slowing down processing of
the wafers. As a result, defect revisiting with SEMs is
usually done olT-line in a quality control or analysis labo 10 against the complex background of patterned wafers, and in
identifying their origins), a variety of software to assist the
ratory.
In an attempt to overcome the limitations of SEMs, some
major semiconductor producers have begun to use systems
which include both low and high voltage SEMs. However,
such systems are expensive, selling in the $1,000,000 to
$1,500,000 range.
15
operator in evaluating and classifying the defect, commu
nications and data storage capabilities for providing trend
analysis on-line or off-line, and capacity for image storage.
For future product line expansion, the laser imaging
SUMMARY OF THE INVENTION
system is adaptable to cluster or in~situ applications, where
examination of defects or structures during on-line process
According to the invention, a laser imaging system that
allows hands-o?~ operation and operates under class 1 clean
BRIEF DESCRIPTION OF THE DRAWINGS
ing can be performed.
room conditions, has several distinct advantages over con
ventional systems for sub-micron particle structure evalua
tion. In one embodiment, the laser imaging system “revisits”
defects on production semiconductor wafers, where the
defects are ?rst detected (but not analyzed or evaluated) by
25
FIG. 1 is a perspective view of a laser imaging system
according to the invention.
FIG. 2 is a schematic diagram of a laser imaging system
according to the invention illustrating the operation of the
conventional wafer scanners such as are available from
laser imaging system.
vendors as Inspex, KLA, or Tencor Instruments, among
FIGS. 3A, 3B, and 3C combined are a schematic diagram
of the electronics associated with the laser imaging system
according to the invention.
others. The laser imaging system replaces and outperforms
conventional microscopes now used to analyze defects on
production semiconductor wafers.
Signi?cantly, the laser imaging system according to the
FIG. 4 is a view of a display screen resulting from analysis
of an area of the surface of a semiconductor chip by a laser
invention is the ?rst defect review tool whose optics and
imaging system according to the invention.
functionality have been designed explicitly for ef?cient
performance of the dedicated revisit task. Unlike scanning
electron microscopes (SEMs) that have previously been
used for defect analysis, the laser imaging system will not
damage samples or slow processing, and costs signi?cantly
35
A laser imaging system according to the invention is used
less to implement than an SEM. Further, while SEMs can
produce images with resolution on the nanometer scale, they
have certain limitations. For example, the SEM image has an
extended depth of ?eld, like a photograph taken through a
high f-stop aperture, but this image contains no quantitative
depth information. Some methods of dealing with this
de?ciency are sample tilting or coating to produce a “shad
owing” effect or perspective change, but these methods
require additional process steps and cost, may damage the
wafer, and do not completely resolve the‘ problem.
Unlike the SEM, the laser imaging system according to
the invention operates in air with class 1 cleanroom com
to analyze defects on semiconductor wafers that have been
40
45
laser imaging system can perform a variety of microscopic
inspection functions including defect detection and metrol
ogy.
50
FIG. 1 is a perspective view of laser imaging system 100
according to the invention. Laser imaging system 100
includes housing 102 made of stainless steel. Laser imaging
system 100 occupies a footprint which ?ts inside a 48"
standard clean hood. Laser imaging system 100 has con
trolled intemal air?ow (clean air from the cleanroom is
rendering techniques, which provides quantitative dimen
55
with correct perspective maintained, without necessity for
sample tilting or coating. Additionally, the laser imaging
drawn in through the top of laser imaging system 100 and
exhausted from laser imaging system 100 outside of the
cleanroom), maintaining class 1 conditions in the wafer area,
which is isolated from the operator console.
A cassette of wafers (not shown) of a given size, e.g.,
system has an ability the SEM cannot match: sub-surface
viewing of defects lying beneath dielectric layers. Combined
with three-dimensional analysis software, the user is able to
examine cross sections of the defect and surrounding mate
rial, and to assess the impact on circuit layers of the wafer.
The laser imaging system presents a real time video image
with resolution superior to a conventional microscope. An
detected by patterned wafer defect detecting systems (wafer
scanners). The laser imaging system replaces optical micro
scope review stations now utilized in the semiconductor fab
environment to examine detected optical anomalies that may
represent wafer defects. In addition to analyzing defects, the
patibility. Also unlike the SEM, the laser imaging system
can produce a three dimensional image, using simple image
sional information. The image can be stored and recalled for
later viewing. The image can be rotated or tilted or shaded,
DETAILED DESCRIPTION OF EMBODIMENTS
OF THE INVENTION
wafers ranging from 3 inches (75 mm) to 8 inches (200 mm)
60
in diameter, is positioned on cassette platform 101. One of
a set of interchangeable mounting plates (not shown), there
being a different mounting plate for each cassette size (i.e.,
wafers of dilTerent sizes are held by different cassettes), is
attached to cassette platform 101. Typically, defects have
operator can view the image on a conventional computer 65 previously been identi?ed on the wafers by a defect detect
ing system, as described above. Wafers from the cassette are
the wafers to operator contamination or air?ow.
loaded through wafer door 103 formed in housing 102 into
display, with comfortable ergonomics, and without exposing
5,479,252
5
6
a wafer processing area housed by optics housing section
precise de-skew alignment, the system can accurately trans
107 of housing 102. The wafers are either loaded by the
operator or by a robot 104 that is part of a standard machine
late any speci?ed location on the wafer into the ?eld of view
of the microscope with an accuracy of a few microns.
The heart of laser imaging system 100 is the laser scan
interface (SMIF) i.e., micro-environmentally controlled,
interface. The SMIF interface, which is a “box” for trans
ferring wafers in which clean room conditions are main
ning microscopic optics module (“optics head”) which
includes elements 201, 202, 203, 204, 205, 207, 208, 210,
211, 212, 219, 220, 221, 222, and 223 shown in FIG. 2
tained, is conventional and is available from Asyst Tech
nologies in Milpitas, Calif. (various models are available
below. The optics head includes a laser, confocal beam
and can be used with the invention). After inspection, the
wafers are unloaded by either the operator or the robot 104.
An optional 3-cassette carousel (not shown) may be
mounted on laser imaging system 100 allowing sorting of
wafers after inspection.
Robot 104 is a conventional precision, high reliability
(less than one wafer drop per million transfers) robotic wafer
handler, such as is available from MECS in Japan as part no.
scanning optics, and ultraviolet and visible photo detection
electronics, together with commercial microscope compo
nents to achieve high quality real time confocal images.
Laser imaging system 100 will produce a complete XY
scanned laser image, in a single plane of focus, at video
rates. The resulting image is displayed on a high resolution
15
UTX~1000. Robot 104 will reliably sense, load, and unload
wafers from cassettes, interchangeably handling 75 mm to
conventional microscope.
200 mm wafers. Robot 104 senses missing or skewed wafers
in the cassette(s), as well as the presence or absence of a
wafer on the robot arm or vacuum chuck 224 (see FIG. 2).
Robot 104 (and other components of laser imaging system
100) is designed to eliminate any wafer contamination (laser
imaging system 100 maintains Class 1 compatible cleanli
ness while handling wafers). Robot 104 has su?icient utility
backup (power, air, vacuum) to protect any wafer in transit
25
on robot 104 from damage. Upon restart after a power
failure, all wafer locations (cassette slots, robot arms,
vacuum chuck 224 (FIG. 2), plus missing cassettes, are
sampled for the presence or absence of wafers and/or
cassettes, and appropriate responses made.
When loading wafers, robot 104 removes wafers from
cassette platform 101 and performs a pre-alignment step,
using pre-aligner 105 which senses a notch and/or ?at(s) on
the wafer. Optionally, any bar code (identifying the particu—
lar wafer) which may be present on the wafer may also be
30
semiconductors. An example of a laser that can be used with
the invention is the Model 2204-25ML air-cooled argon ion
35
interference problems that can occur for a speci?c wave
40
45
(such as Helium-Neon or Helium Cadmium) could also be
mounted on a conventional computer controlled ?lter wheel
(not shown) within optics housing section 107. Notch ?lter
202 isolates a laser line or lines. A notch ?lter for use with
the invention is available from Edmund Scienti?c of Bar
rington, NJ. as part no. 43120. Other ?lters are available
55 from the same source for other wavelengths.
The light having the selected wavelength(s) passes from
notch ?lter 202 to polarizing beam splitter 203. Polarizing
various aspects of the system. The X and Y axes de?ne a
beam splitter 203 is attached to selectable notch ?lter 202
60
using conventional optical mounts. Polarizing beam splitter
203 preferentially re?ects light only of the proper polariza
tion and directs the light to spatial ?lter 204. The polariza
tion of the light emitted from laser 201 is oriented so that
most of the light is re?ected by polarizing beam splitter 203
at 90 degrees into the focusing optics of spatial ?lter 204. A
for the mis-orientationof the patterns on the wafer with
respect to the wafer ?at and the robot 104 placement error),
and automatically focuses using the laser as described
scope ?eld of view with etched ?ducial marks or other
pre-speci?ed structures on the wafer surface. After this
wavelengths strongly, but will allow transmission of others
to perform the imaging desired. Other wavelength lasers
used to supply light at other wavelengths.
Laser 201 produces polarized light at several discrete
wavelengths. The light passes through a “notch” ?lter 202
bay 106 of housing 102.
below. The operator accomplishes ?ne alignment of the
wafer (dc-skew point) by lining up the visible light micro
give good results for all materials, ?lm thicknesses, and
surface properties. Additionally, in many cases, it is desir
able to observe through one or more top layers of material
(typically dielectric) which will re?ect or absorb some
a link or network such as Ethernet, RS232, etc. A computer
plane parallel to the patterned surface of the wafer and the
Z-axis is perpendicular to the patterned surface of the wafer.)
The system translates the wafer to the ?rst “de-skew” point
(i.e., pre'determined orientation of the wafer that accounts
laser produced by Uniphase Corporation, San Jose, Calif. It
is important to perform imaging with a selection of wave
lengths of laser light to overcome absorption, re?ection, and
length for a given material. That is, one wavelength will not
either by diskette or other media, or by communication via
After pre-alignment, the wafer is loaded into the wafer
processing area through wafer door 103 onto the optical
unit’s XYZ-stage (translational motion) and is secured on a
conventional vacuum chuck 224 (FIG. 2). (In this descrip
tion, a Cartesian coordinate system is used in description of
sample at which it is desired to obtain imaging data.
Laser imaging system 100 includes an air cooled, multi
line argon ion laser 201 which provides up to six different
wavelengths of light for imaging surfaces and structures in
While the wafer is being loaded, a ?le of data from the
for use with laser imaging system 100 is available from
Silicon Graphics in Mountain View, Calif. as part no. SGI
XS24Z. A disk drive, available from Silicon Graphics in
Mountain View, Calif. as part no. P3-F252, and tape drive,
available from Hamilton/Avnet in Mountain View, Calif. as
Maynard (Archive) 215018, are attached within disk drive
FIG. 2 is a schematic diagram of laser imaging system
100 according to the invention illustrating the operation of
laser imaging system 100. Laser imaging system 100 uses
the basic principles of confocal microscopy, in which illu
minating light passes through a pinhole, and the image of
this pinhole is then cast by the system optics on the sample
to be viewed. The light scattering from the sample returns
through the system optics to the pinhole, but only light from
the focal plane of the imaging (objective) lens returns
through the pinhole, i.e., light from the plane through the
read at this time.
defect detecting system, specifying the wafer coordinates of
the detected defects, is transferred to the laser imaging
system computer within housing 102 (not visible in FIG. 1),
monitor, also in real time. Thus, the operator can scan
through different levels of focus in real time, as with a
65
small portion of the light passes through polarizing beam
splitter 203 to a conventional power monitor diode (not
shown) mounted behind polarizing beam splitter 203, where
5,479,252
7
8
the light is absorbed. A polarizing beam splitter for use with
the invention is available from Melles Griot of Irvine, Calif.
objective lenses 205 and autofocus (one lens is focused and
focus o?’sets stored in the computer are used to automati
cally focus the other lenses) of each objective lens 205. A
as part no. 03PBB003.
turret for use with the invention is available from Olympus
of Japan as part no. BL0920. Turret 223 is designed to
accommodate three to six objective lenses 205, and can
Spatial ?lter 204 consists of optics which expand the
beam and then focus it on a pinhole aperture. The diameter
of the pinhole aperture is selected according to well-known
techniques to re-image the light through the downstream
optics and a selected one of a plurality of objective lenses
205 to produce a diffraction-limited spot on wafer 206. The
diameter of the pinhole aperture is also selected to allow
easy alignment of the beam of light and a signi?cant amount
10
of high power light to pass through the aperture. A spatial
?lter for use with the invention is available from Melles
Griot of Irvine, Calif. as Compact Spatial Filter Newport/
910. Spatial ?lter 204 is attached to polarizing beam splitter
203 by conventional optics mounts.
Subsequent optics within spatial ?lter 204 and between
the pinhole assembly and the scanner mirrors collimate the
tube lens 211 as a standard component are mounted together
with a ?ange and held by a locking screw. The turret]
illuminator assembly bolts to the optics baseplate.
According to the principles of confocal imaging, the light
striking wafer 206 is scattered and a portion of the light
light, and direct the light to mirrors mounted on X-Y beam
scanner 207. An X-Y beam scanner for use with the inven
20
tion is available from General Scanning of Watertown,
Mass. as part no. GOO-3011003. X-Y beam scanner 207 is
attached to spatial ?lter 204 by conventional optics mounts.
The mirrors in X-Y beam scanner 207 can oscillate their
angle with respect to the beam of light passing through X-Y
25
operating at 8 kHz, the other a servo controlled unit,
on the sample, very little light returns through the aperture.
operating at 13 or 26 Hz (but capable of other speeds). The
30
aperture reaches the polarizing beam splitter 203, which,
being oppositely polarized, passes through polarizing beam
35
tern and allowing more light to reach the tube lens 211,
45
below, are standard in?nity corrected optics.
Quarter wave plate 210 is attached to scan lens 208 and
is positioned to convert the linearly polarized laser light to
circularly polarized laser light. A quarter wave plate for use
with the invention is available from Melles Griot of Irvine,
the raster scan, a map of light intensity in the focal plane of
the objective lens 205 is constructed. This map can either be
stored in the memory of system computer 214, or analyzed
by surface data processor 213, which stores the readings,
and makes a comparison of the intensity with previously
stored maps from other scans, as described below. The light
intensity map is also written directly into the video memory
of the system computer 214 and may be displayed live on the
computer display 215 in an appropriate window, as
described below.
To obtain a three dimensional image, the optics head
works with the ?ne z-stage control 216 to develop an
expanded depth-of-?eld image. The sample height is
50
stepped over a pre—selected vertical interval (typically 12 nm
or some multiple thereof) using the ?ne z-stage control 216.
After each complete raster scan at a particular sample height,
Calif. as part no. 02WRM005. Beam splitter 209 is attached
to quarter wave plate 210 by a conventional optical mount,
and is explained in more detail below. Tube lens 211 is
attached to beam splitter 209 by a conventional optical
mount and works with objective lens 205 to de-magnify the
raster scanned pinhole image and project it on the wafer 206.
splitter 203 undeviated and is imaged on the photodetector
212.
By measuring the light intensity at each XY location of
providing a more uniform brightness across the raster pat
described below, without distorting the image. The tube lens
Consequently, signals in the confocal optics get darker, not
merely blurred, as occurs with conventional optics, when the
sample is out of focus. Light which passes through the
scanner 207 traces out a raster pattern in space. A raster scan
211 and objective lenses 205, described in more detail
the ?eld lens, scan lens 208, and mirrors of X-Y scanner 207
204. If the light spot was in focus on the sample, the image
is imposed on the aperture. If the light spot was out of focus
oscillating galvanometers, one a high speed resonant unit
of 256 or 512 lines is produced at approximately 26 or 13
frames per second, and is imaged at the back focal plane of
the tube lens 211.
This raster pattern is imaged in space by the scan lens 208
in the plane of the ?eld lens (not shown, but between beam
splitter 209 and quarter wave plate 210). A scan lens for use
with the invention is available from Applied Optics of
Pleasanton, Calif. as part no. 000424. Scan lens 208 is
attached to X-Y beam scanner 207 by conventional optical
mounts. The ?eld lens serves to collect high angle light,
reflected back into objective lens 205, returning through the
optical path described above. As the retuming light passes
through quarter wave plate 210, the returning light is con
verted to light linearly polarized and 90” out of phase with
respect to the polarization of the light originally emitted by
laser 201. The light continues back along the path through
until the light reaches the pinhole aperture of spatial ?lter
beam scanner 207. X-Y beam scanner 207 includes two
servo steps in small increments, so that the X-Y beam
handle low power (magni?cations of 5, l0 and 20 times
actual size) as well as medium power (magni?cation of 50
times actual size) and high power, high N.A. (numerical
aperture, a conventional designation for the light gathering
property of an objective lens in which higher numbers
indicate a broader cone of gathered light) objective lenses
205 (magni?cations of 100 and 150 times actual size and
0.95 NA). Turret 223 and a vertical illuminator containing
the height of the sample is changed using ?ne z-stage control
216, and a new raster scan perfonned, as described above,
55
to obtain a map of light intensity in the focal plane of
objective lens 205 (at the new sample height) by measuring
the light intensity at each XY location of the raster scan.
A tube lens for use with the invention is available from
X-Y stage control 218 is used to position the defect or
Olympus of Japan as part of their vertical illuminator model
region of interest in the ?eld of view. The X-Y stage control
SLM220.
60 is then held still while the ?ne z-stage control 216 is used as
described above.
The image of the light spot is focused and demagni?ed by
the objective lens 205 in the focal plane of the objective lens
A three-dimensional image can be obtained from the
205. Objective lenses 205 for use with the invention are
multiple XY light intensity maps in one of two ways. First,
available from Olympus of Japan by specifying l00xBF
l-LM590. Many interchangeable lenses are available.
Objective lenses 205 are mounted on a computer controlled
motorized turret 223 that enables automatic changing of
as noted above, the XY data from each raster scan can be
65
analyzed by surface data processor 213 by comparing the
light intensity at each point of the XY scan with correspond
ing points of a “master map.” This “master map” stores the
5,479,252
10
maximum light intensity values found at each XY point,
these values resulting from previous comparisons of XY
light intensity maps. The Z-axis location of the maximum
light intensity at each XY location is also stored. After all of
the XY light intensity maps have been obtained and com
pared to the “master map,” the data representing the light
intensity maximumat each XY location and the Z-axis
location of each light intensity maximum are used to con
struct the three-dimensional image of the wafer surface.
With this method, it takes about 5 seconds to acquire all of
the light intensity data and extract the surface.
Alternatively, especially if the wafer is multilayered, i.e.,
producing multiple peaks at each XY location along the
Z~axis (which might occur, for instance, where transparent
layers are formed), each light intensity map can be stored in
system computer 214, along with the Z-axis height of each
lengths pass to the video camera, a conventional microscope
image can be obtained, in addition to the laser image, by
using a conventional microscope illuminator 220 and video
camera 219, charge coupled device (CCD). The white light
imaging is accomplished without the use of microscope
eyepieces that would result in undesirable proximity of the
operator to the wafer being analyzed that may result in
contamination of the wafer. Rather, the microscope image is
displayed on a computer display (simultaneously with the
10
computer display 215, using software described in more
detail below, or on a separate video monitor display (not
shown).
15
The white light microscope image is produced alone or
simultaneously with the live laser image by video camera
219 available from COHV of Danville, Calif. as part no.
8215-1000 which views the sample in white light emitted by
microscope illuminator 220, and inserted into the optical
path by beam splitter 221. A microscope illuminator for use
map. If it is desired to create a three-dimensional image of
the surface of the wafer, the XY light intensity maps are
successively compared to determine the maximum light
intensity at each XY location. The Z-axis location of the
maximum light intensity at each XY location is stored and,
at the conclusion of the series of comparisons of the XY light
intensity maps, is used with the maximum light intensity
laser image, if desired), either in a separate window on
20
with the invention is available from Olympus of Japan as
part no. 5LM220. A beam splitter for use with the invention
is available from Melles Griot of Irvine, Calif. as part no.
O3BSC007. Filter 222 blocks the laser line in use, but passes
broad bands of light having other wavelengths, so that laser
light from laser 201 is prevented from saturating the image
data to construct the three-dimensional image of the wafer
surface. With this method, it takes about 35 seconds to 25 at video camera 219 with re?ected light. A ?lter for use with
the invention is available from Edmund Scienti?c of Bar
acquire all of the light intensity data, then extract the surface
rington, NJ. as part no. 22754. Video camera 219 and ?lter
using a processor in system computer 214.
wheel 222 are mounted on brackets which position video
A process for constructing a three-dimensional image of
camera 219 and ?lter 222 in line with beam splitter 221.
a surface from a three~dimensional data set is described in
Beam splitter 221 is mounted on the turret assembly with
more detail in commonly owned, co-pending US. patent
conventional optical mounts.
application entitled “Surface Extraction from a Three-Di
mensional Data Set,” by Ken K. Lee, application Ser. No.
To get a white light image alone, laser imaging system
08/079,193, ?led on the same date as the present application,
100 can remove beam-splitter 221 and substitute a mirror
the pertinent disclosure of which is hereby incorporated by
(not shown) so that only the video camera light path is
35 active. Then, the blocking ?lter (mounted on a ?lter wheel)
reference.
can be removed and the full spectrum white light image
The raster scan is repeated 13 times per second for a 512
viewed.
by 512 pixel image, or faster for smaller (i.e., fewer pixels
FIGS. 3A, 3B, and 3C combined are a schematic diagram
such as 256 by 256) images. (Note that raster scan sizes
of the electronics associated with laser imaging system 100.
other than 512 by 512 or 256 by 256 can be used.) A
complete three~dimensional volume data set will typically
40
total data array of size 512 by 512 by 64. For a 512 by 512
pixel image, the total time to accumulate the data to con
struct the three—dimensional image of the surface (assuming
FIGS. 3A, 3B and 3C show all analog and digital electron
ics, plus power supplies, for complete operation of laser
include 64 raster scans (other numbers can be used), for a
imaging system 100. Laser imaging system 100 operates on
220 volts (200-240 volt nominal), 50/60 Hz single phase
45
electric power (or the European and Japanese equivalents).
The SDP Frame Grabber 301 interfaces with photo detec
tor 212 (FIG. 2) and is synchronized with the scanner
electronics 309, and ?ne z-stage control 310, to digitize the
The light intensity at each data point is stored in system
photodetector data and produce a three-dimensional map of
computer 214 as an 8-bit quantity. A simple map of a
three-dimensional surface is created using the three-dimen 50 light intensity which can either be stored directly in the
64 raster scans, i.e., vertical height steps) is approximately
?ve seconds.
computer memory, or processed to immediately extract a
sional graphics (such as the Silicon Graphics Inc. Graphics
Library, available as part of the XS24Z computer package)
surface image. The SDP Frame Grabber 301 interfaces
through the SDP interface 302 to the system computer 303
(also shown as 214 in FIG. 2). SDP Frame Grabber 301 is
of system computer 214 by plotting the X, Y, and Z position
of each maximum intensity point, and displaying the map as
a continuous surface. The brightness of each point on the
55
surface is determined by the light intensity measured at that
point. The map display may be done in gray scale, in false
fast and enables surface data to be extracted from the volume
data.
The system computer 303 is a high speed RISC graphical
workstation, such as a Silicon Graphics Iris Indigo XS24Z
color converted from the gray scale, in a mode showing
shape (position) only, or shape with height represented in
manufactured by Silicon Graphics of Mountain View, Calif,
gray scale.
or equivalent, capable of handling concurrent tasks of robot
functions, stage motion, operator interface, and optics con
The capacity for white light imaging, in addition to the
laser imaging described above, is another feature of laser
trol, while also performing image processing functions. In
imaging system 100. ‘As noted above, beam splitter 209 is
attached between quarter wave plate 210 and tube lens 211.
By imposing beam splitter 209 in the path of light from laser
201 just prior to tube lens 211, and using suitable ?ltering
that blocks the re?ected laser light but lets other wave
65
addition, system computer 303 must work with a windowing
user interface and high resolution color graphics.
The X, Y, and coarse Z stage controllers 304 communicate
with system computer 303 via an RS-232C interface 305, as
do the robot and pre-aligner controllers 306.
5,479,252
11
12
The balance of the system electronic functions commu
The user can utilize laser imaging system 100 as a white
nicate through Local Operating Network (LON) interface
light microscope. The microscope view is presented in a
different window than the laser view. In white light mode,
the user can select views (e.g., two-dimensional), translate
(X-Y movement) and focus on details (Z-direction move
307 built on the same interface slot as RS-232C interface
305. The LON 308 itself is a pair of wires that plug into each
node serially around the system. Each node contains a local
processor and ?rmware for LON communications, self diag
nosis, and local operation of certain functions.
ment), change objective lens magni?cation.
The overall system software is designed for operation of
laser imaging system 100 in both operator and engineering
All user interface is via an operator console that is part of
system computer 303 and which includes computer display
215, a mouse/trackball, a joystick controller, and a keyboard.
10
mode, optionally using defect ?les supplied by various
wafer scanners in a variety of formats. Both operator and
The operator console may optionally be remotely mounted
engineering mode are password protected separately, as
(i.e., outside the cleanroom). Image processing and analysis
explained in more detail below.
In engineering mode, the user can use a recipe develop
ment editor to develop recipes for routine inspection of
functions may be controlled from the console. Through these
controls and the windowing software, the operator can set
up, program and operate any part of laser imaging system
speci?ed types of wafers at speci?c process steps for that
wafer, i.e., to pre-specify operating parameters for use by
100 including wafer selection and handling, defect editing
and selection, automatic and/or manual wafer loading,
defect classi?cation, etc. For example, the joystick control
operators working on a speci?c process level and product.
The recipes can specify which screen and windows are to be
used, enable the laser wavelength and power to be used to
be selected, speci?cation of the number of slices of data and
ler allows the user to move the coarse Z-stage control 217 in
small increments, to bring an object or region of interest on
the wafer into view. Alternatively, for enhanced ease in
making very small lateral movements, the operator can use
their spacing (in nanometers), autofocus to be speci?ed, and
the mouse to point and click to cause the X-Y stage control
218 to change position.
25
Engineering mode also allows access to system mainte
The operator has three modes from which to select
nance functions such as utilizing the LON access to run
viewing: white-light conventional microscope optics
(“white light mode”), real time laser scanning optics (“laser
diagnostic checks on the electronics or recalibrate XY and Z
stage motion
In operator mode, the operator loads, inspects and clas
si?es lots of wafers per the predetermined recipe associated
with the particular lot number and wafer ID. The operator
has limited options to alter the inspection sequence.
mode”), or both laser and white light optics simultaneously
(“combination mode”). In white light mode, the operator can
select from one of several objective lenses, varying effective
magni?cation of the image. (The laser image scales simul
taneously with the white-light image.)
Laser or white light imaging of a region of the wafer
the offset in the z-direction (vertical direction) from the
autofocus position to the ideal viewing position to be preset.
35
Utilities are available in pop-up menus to enable manual
produces data regarding the wafer characteristics in the
imaged region. The data is stored on system computer 214.
control of the ?ne Z-stage control 216, coarse Z-stage
control 217 and X-Y stage control 218, polling of stage
After imaging of a region of the wafer, the operator exam
ines each defect image as laser imaging system 100 presents
it to him. If the defect is not in view (if, for instance, the
ment of a wafer from the ?at ?nder or stage after a power
variables, and robot manual control (e.g., to allow move
40
defect location data from the wafer scanner is slightly
erroneous), or the operator wishes to examine a larger or
different area, the joystick controller allows him to “cruise”
the wafer. After the operator examines the defect, the opera
tor classi?es the defect, optionally records the image, and
proceeds to examine the next defect. Upon completion of
review of all desired defects, or other inspection tasks, the
display of LON nodes, direct control of system functions,
45
store wafer images and to bring them up in Library win
dows. These images are usually stored as bit maps and can
50
of the screen of computer display 215, as explained in more
detail below, the operator sees a real-time narrow depth of
?eld laser image, which may be zoomed to higher e?‘ective
55
re?ectivity of the wafer) may also be employed to automate
most of the operator’s tasks in acquiring the three-dimen
sional image. The three-dimensional image may be exam
ined in pseudocolor, pro?led, shadowed, rotated, etc.
for comparison between inspection systems, thus enabling
the operator to compare the image produced by laser imag
ing system 100 with familiar images. The bit maps can be
displayed in special windows to help operators classify
matic ranging (automatic selection of vertical distance to
traverse in obtaining imaging data), automatic focus (auto
matic focusing of objective lens at desired vertical location)
and automatic gain control (automatic adjustment of the
photodetector gain to compensate for differences in the
be used to represent typical defects for comparison by the
operator with the defect currently being classi?ed. Laser
imaging system 100 can be con?gured to bring up such
images automatically as di?°erent defect classi?cations are
selected. Bit map images from other devices, such as white
light microscopes or SEMs can also be stored and recalled,
magni?cations. The operator can translate the sample in the
Z-direction (vertical direction) to cover an entire vertical
region of interest. The operator can also select a range of
vertical motion, and have laser imaging system 100 con
struct a 3-dimensional image of the region speci?ed. Auto
e.g., open and close the laser shutter, can be accomplished.
As an aid to workers reviewing defects (especially during
production), laser imaging system 100 has the capability to
wafer is returned to its cassette, and the next wafer loaded,
repeating the process above.
In the live laser image, presented in a particular window
failure). Diagnostics can also be called up via a pop-up
window displaying all LON nodes and system variables. The
status of LON nodes can be checked and revised. From the
60
65
defects.
The system software includes different levels of password
protection. At one level, engineering personnel can access
laser imaging system 100 to set up predetermined recipes for
screen con?guration, laser scan parameters, and defect
codes. At a different level, operators can access laser imag
ing system 100 to call up the recipe for the particular wafer
level and product being used in order to examine defects on
wafers to be inspected. This feature, combined with auto
5,479,252
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14
matic focus, ranging, and gain control, allows competent
operation of laser imaging system 100 with a minimum of
The Laser Window directly displays the live laser image
produced by the scanning laser beam. Controls for changing
operator training.
the focus of the laser through its range are available.
Laser imaging system 100 uses an Ethernet interface that
supports standard ?le transfer and management (data and
number of slices to utilize for a three-dimensional image,
Additional controls include autofocus, laser intensity, zoom,
recipe upload/download, etc.). Laser imaging system 100
and the step size of three-dimensional image slices, plus
other imaging control features.
includes software for generating output report ?les for use
by data analysis (trending, statistical analysis, etc.) software
The 2-D Window utilizes the acquired three-dimensional
surface image, and presents a two-dimensional image of a
slice through the three-dimensional data. In “XY” mode, the
2-D Window displays a top view in false color of the Wafer
as well as printed reports. A number of confocal images may
be stored in ?les for subsequent review on or off line from
laser imaging system 100.
As noted above, laser imaging system 100 includes com
surface, i.e., a projection of an effectively in?nite depth of
?eld image of the wafer surface. In “X2” and “Y2” modes,
the surface image is represented as surface pro?les of
selected vertical slices. In the two-dimensional images, a
rubber band metrology box (i.e., a variable size cursor box)
can be used to determine the size (in the plane chosen) of the
puter display 215. Computer display 215 displays pictorial
and numerical results of the analysis of the defects on the
semiconductor chips, and lists menu selections for control of
laser imaging system 100. Laser imaging system 100 pro
vides the capacity for displaying any number of different
display screens on computer display 215. Each display
defect. In “cut” mode, the “ Y” mode view includes cursor
lines which can be controlled by, for instance, a mouse to
20 select “XZ” mode or “YZ” mode slices. If the 2-D Window
of windows that can be displayed on a display screen is
is used with a volume data set rather than a surface data set,
screen is de?ned by the number, type, size and location of
the windows included within the display screen. The number
limited only by the size of the windows to be displayed.
Generally, the types of windows that can be displayed on
each screen are icon windows, picture windows and infor
mation windows.
FIG. 4 is a view of display screen 400 resulting from
analysis of an area of the surface of a semiconductor chip by
laser imaging system 100. Display screen 400 includes a
the “XY” mode display shows a single slice scan of data,
rather than a surface outline. The “XZ” mode and “Y2”
25
mode show vertical cuts through the data set. Special options
allow pro?les of volume data sets to show multiple layer
structures. This occurs by analyzing volume data from
multilayer semi-transparent samples.
The 3D Window projects a perspective view of the
surface image. The 3D Window may be rotated, tilted,
zoomed, shaded, etc. by the operator to obtain a desired
plurality of windows of various types. In FIG. 4, windows
401a, 401b, 401e, 401d, 401e, 401f and 401g are icon
windows; windows 4020, 402b, 4020 are picture windows;
image for analysis.
and window 403 is an information window.
In FIG. 4, the icon windows list choices for pictorial
The Wafer Map Window displays the defect map of the
wafer under inspection, the defect map having been pro
display in the picture windows. For instance, icon windows
duced by a wafer scanner that is not part of laser imaging
401a and 401d command display of a two-dimensional 35
image in one of the pictorial windows, e.g., the surface
pro?le seen in window 402b or the planar surface view seen
in FIG. 402a. Icon window 401a commands display of a
three-dimensional image in one of the pictorial windows,
e.g., the three~dimensional surface image seen in window
402C. The pictorial windows 402a, 402b and 402C show two
or three-dimensional images of the semiconductor chip
being analyzed, as discussed above. The information win
dow 403 gives tabular information regarding the size and
location of particular defects on the chip and is describe in 45
more detail below.
Laser imaging system 100 includes a number of pre
de?ned screens (in one embodiment, on the order of 4-5
list.
The Cassette Map Window diagrammatically represents a
loaded cassette. The operator can select and load any wafer
from the cassette by, for instance, using a mouse to “point
and absorption. It is always a problem to focus microscopes
55
pictorial window is stored as an icon window when not in
use. Windows in different regions of the screen may be
,
through and selected directly by highlighting items on the
There is a major advantage to using a laser with multiple
lines to image surfaces, to account for different re?ectivities
to de?ne an unlimited number of screens, each screen having
interchanged by “clicking and dragging” the window to the
pop-up text window (the defect locations are given as
coordinates of a Cartesian coordinate system), scrolled
imaging system 100.
laser imaging system 100 includes the capability for a user
new location.
defect. A rubber band metrology box may be used to display
a portion of the defect map to higher precision in an enlarged
view. Alternatively, a list of defects can be brought up in a
and click” on the wafer. The operator can also unload the
wafer to a cassette on the cassette platform 101 of laser
screens), i.e., screens having windows of a pre-determined
type and size located at pre-determined locations. However,
any desired combination of windows according to window
type, window size and window location.
The screen (window and arrangement of windows) to be
displayed is selected by the operator. For each screen, each
system 100. Defects are shown as a color-to-size coded dot
on the screen. The operator can select a defect to revisit by,
for instance, using a mouse to “point and clic ” on the
automatically. Confocal optics are a natural way to perform
an autofocus due to the extremely narrow depth of ?eld.
However, a single spot autofocus may not work well if a
very dark spot on the sample is being imaged. By scanning
the laser spot in real-time, and averaging over the return
60
signal, a much more reliable autofocus is obtained. If a
particular material is strongly absorbing, different wave
lengths may be selected. Alternatively, a laser ?lter may be
used which transmits each wavelength of the laser inversely
proportionally to the intensity of the wavelength and so
65 illuminates the sample with multiline laser light which is
sure to select at least one wavelength which re?ects strongly.
in white-light only mode for optimum viewing of the
' white-light image.
In a simpler mode, the laser can be used with no ?lter, and
The Microscope Window contains a live video presenta
tion of the white-light image taken from the wafer surface
through the microscope objective lens. This image may be
viewed simultaneously with the laser image or can be view
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16
the computer can adjust laser power and photodetector gain
In its simplest mode, laser imaging system 100 works on
opaque, diffusely scattering surfaces.
to achieve a good autofocus.
False color can be used to see small defects, either on the
surface or under the surface. By converting images to false
color from black and white, very small objects may be
detected that would otherwise go unnoticed, because the
human eye can discern thousands of separate hues, while
small differences in brightness are hard for the eye to detect.
This is one way in which laser imaging system 100 exceeds
the “Rayleigh Criterion” (a common measure of resolution)
in its ability to see structures on the order of 0.1 to 0.2
micron size. Other aids in small feature detection are the
The laser imaging system according to the invention is
unique in its capability for combining live video, live laser
scan, three-dimensional imaging, wafer maps, and wafer
surface pro?les all on one screen. By selecting screens with
different size and composition of windows the operator may
view all the relative data needed for completion of his task.
The system according to the invention can be easily
10
displays, etc. The necessary adaptations to laser imaging
improvement in resolution obtained with confocal optics,
system 100 would include some software changes and some
and the oversampling (overlapping images) performed by
changes in the material handling system.
Appendix A accompanying this speci?cation is a draft
laser imaging system 100 that enables detection of features
as small as 0.1 to 0.2 rrricrons.
User’s Manual for laser imaging system 100 and is herein
As an option, the laser optics can be used to obtain a
incorporated by reference.
?uorescence image of the wafer, detecting visible light in
preselected wavelength bands. Fluorescence using white
light or laser light in the same system is possible. Illumi
nating light of selected wavelengths will ?uoresce certain
20
materials, such as skin ?akes or photoresist, the material
emitting light at a longer wavelength. Suitable ?lters in front
We claim:
length, but pass the longer wavelength, thus enabling iden
25
1. An imaging system comprising:
means for inspecting a semiconductor die to determine the
e?icacy of a process previously performed on the
semiconductor die, the means for inspecting compris
mg
a multiline laser light source that emits laser light of a
?lter passing the longer wavelengths in front of the photo
detector 212.
Polarized light images of objects or structures on the
sample may be taken by illuminating the objects or struc
plurality of wavelengths,
tures with polarized white light and viewing the sample
through a cross polarized ?lter. Optically active materials
such as quartz will appear bright against the dark polariza
tion extinction background. Optional bright?eld/dark ?eld
objective lenses and illumination may be used to help locate
particles and other defects on the wafer surface.
The same technique may be used with the confocal optics,
Various embodiments of the invention have been
described. The descriptions are intended to be illustrative,
not limitative. Thus, it will be apparent to one skilled in the
art that certain modi?cations may be made to the invention
as described without departing from the scope of the claims
set out below.
of the video camera block the illuminating radiation wave
ti?cation of the material. This may also be done with the
laser, ?uorescing with a short wavelength, and placing a
adapted for use in other materials science industries such as
production of magnetic media, thin ?lm heads, ?at panel
means for directing the laser light toward the semicon
ductor die, and
35
means for measuring a ?rst intensity of laser light
re?ected from the semiconductor die; and
means for analyzing a defect on the semiconductor die to
determine the nature and origin of the defect.
2. An imaging system as in claim 1, wherein the means for
by using linearly polarized light. Inserting a second quarter
analyzing further comprises means for determining at least
wave plate in the beam adds another 90 degree rotation, so
one material of which the defect is constituted.
that re?ected light from the sample no longer passes to the 40
3. An imaging system as in claim 1, wherein the means for
detector, but light further rotated by passing through the
analyzing further comprises means for determining at least
optically active medium is allowed to pass and shows as a
bright spot on the live laser scan.
one dimension of the defect.
A second major use of conventional microscope stations,
different from the review or “revisit” function, is the more
45
general defect detection function, where preselected sites on
a wafer are inspected for e?icacy of a previous process step.
Laser imaging system 100 according to the invention is
directly usable for this application, which faces exactly the
same problems as defect imaging: decreasing size of objects
of interest, a lack of resolution, and the need for three
dimensional imaging. The hardware required is exactly the
tion of the defect on the semiconductor die.
5. An imaging system as in claim 1, wherein the laser
emits one of said plurality of wavelengths at a second
intensity, and wherein the means for inspecting further
comprises a second ?lter that allows said one of said
plurality of wavelengths to be transmitted to the semicon
ductor die with a third intensity inversely proportional to the
second intensity.
same, as is the system control software. Only a variation in
the application software is needed, so that predetermined
inspection sites may be speci?ed instead of defect map sites.
Overlay registration is another possible use of laser imag
ing system 100. Laser imaging system 100 can view pho
toresist layers on top of underlying structures to ascertain
accuracy of placement of the photoresist layer with respect
4. An imaging system as in claim 1, wherein the means for
analyzing further comprises means for determining a loca
55
6. An imaging system as in claim 1, further comprising:
means for supporting the semiconductor die;
means for isolating the semiconductor die within a region
of the system, and wherein air within the region con
forms to Class 1 cleanroom conditions.
7. An imaging system as in claim 6, further comprising
means for remotely loading and unloading the semiconduc
to features of the underlying structures.
Laser imaging system 100 can also be used to view
tor die onto and off of the means for supporting such that air
within the region conforms to Class 1 cleanroom conditions.
through transparent (e.g., dielectric or glass) layers to allow
8. An imaging system as in claim 1, wherein the imaging
determination of the vertical site of defects, or to provide
system can resolve features on the object as small as
some specialized inspection. For example, stress voids in 65 approximately 0.1 microns.
metal layers below dielectric layers can be viewed. The
9. An imaging system as in claim 8, wherein the laser light
depth of metal plugs in glass insulators can also be seen.
source is an ultraviolet laser.
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17
20. An imaging system as in claim 11, wherein:
10. An imaging system as in claim 8, wherein images are
rendered in false color.
the white light is polarized; and
the means for imaging an object using white light further
11. An imaging system comprising:
a laser imaging means for imaging a surface of an object
comprises means for allowing only light re?ected from
using laser light, the laser imaging means comprising
optically active material in or on the object to pass
through to the means for measuring the intensity of
a laser light source that emits a beam of laser light;
a spatial ?lter for focusing the beam on a pinhole
white light.
aperture;
21. An imaging system for detection or analysis of defects
a lens arranged between the pinhole aperture and the
surface, the lens having a focal plane;
means for scanning the beam and directing the scanned
beam toward the object through the lens; and
means for measuring the intensity of laser light
re?ected from the object and back through the lens
and the pinhole aperture, wherein the measured
intensity of laser light re?ected from the object is at
in or on an object, the system comprising:
means for producing light of a ?rst polarity;
means for directing the light of a ?rst polarity through a
pinhole aperture and toward the object such that a
portion of the light of a ?rst polarity is re?ected by a
surface of the object;
a lens mounted between the pinhole aperture and the
surface, the lens for focusing the light of a ?rst polarity
a maximum intensity when the surface lies in the
to a focal point on or within the object;
focal plane of the lens; and
a white-light imaging means for imaging a surface of an
object using white light, the white-light imaging means
20
comprising
is at a maximum intensity when the surface and the
a white light source that emits white light;
means for directing the white light toward the object;
means for measuring the intensity of white light
re?ected from the object; and
a ?lter that prevents laser light from impinging upon
the means for measuring the intensity of white light
25
focal point of the lens are coincident;
means for changing the polarization of the light re?ected
by the object so that the polarization of the light
re?ected by the object is of a second polarity different
than the ?rst polarity;
means for ?ltering the re?ected light so that the re?ected
and allows white light to impinge upon the means for
measuring the intensity of white light;
wherein the laser light imaging is performed at the same
time as the white light imaging.
12. An imaging system as in claim 11, wherein one of the
laser light or the white light is emitted at a ?rst wavelength
means for measuring the intensity of the light re?ected
from the object and back through the pinhole aperture,
wherein the intensity of light re?ected from the object
30
light passing through the means for ?ltering has an
intensity directly proportional to the distance between
the focal point of the lens and the surface; and
means for analyzing the intensity of to create an image of
the object.
that causes ?uorescence of a particular material in or on the
22. A laser imaging system for imaging a surface of an
?lter, and prevents light of the ?rst wavelength from passing
through the ?lter.
pinhole aperture and output a scanned beam toward the
13. An imaging system as in claim 11, wherein:
the means for imaging an object using laser light further
a lens arranged between the beam scanner and the surface,
comprises means for displaying the laser light image;
a sensor generating an output signal proportional to the
object such that the one of the laser light or white light is 35 object, the system comprising:
a laser light source that emits a beam of laser light, the
re?ected from the material at a second wavelength, and
beam focussed on a pinhole aperture;
wherein the imaging system further comprises a ?lter that
allows light of the second wavelength to pass through the
a beam scanner con?gured to receive the beam from the
and
the means for imaging an object using white light further
comprises means for displaying the white light image.
14. An imaging system as in claim 13, wherein an image
surface;
the lens having a focal plane; and
45
of the object can be displayed as a two-dimensional view of
a surface of the object.
50
15. An imaging system as in claim 14, wherein an image
of the object can be displayed as a two-dimensional view of
a slice through the object.
16. An imaging system as in claim15, wherein an image
intensity of laser light re?ected from the object and
back through the lens, the beam scanner, and the
pinhole aperture to impinge on the sensor, wherein the
output signal of the sensor indicates a maximum inten
sity of re?ected laser light when the surface lies in the
focal plane of the lens.
23. The imaging system of claim 22, wherein the laser is
a multiline laser light source that emits laser light at a
plurality of wavelengths.
24. An imaging system as in claim 23, wherein the laser
of the object can be displayed as a three-dimensional view 55 emits one of said plurality of wavelengths at a ?rst intensity,
of a surface of the object.
and wherein the means for inspecting further comprises a
17. An imaging system as in claim 16, wherein at least
?lter that allows said one of said plurality to be transmitted
two of the views can be displayed simultaneously.
18. An imaging system as in claim 11, wherein:
to the semiconductor die with a second intensity inversely
proportional to the ?rst intensity.
25. An imaging system as in claim 23, wherein the beam
travels along a ?rst path between the laser and the surface,
the system further comprising:
a beam splitter arranged in the ?rst path of the beam
the laser light is polarized; and
the means for imaging an object using laser light further
comprises means for allowing only light re?ected from
optically active material in or on the object to pass
through to the means for measuring the intensity of
laser light.
19. The imaging system of claim 11, wherein the scanning
means comprises a resonant mechanical scanner.
between the laser and the lens, the beam splitter con
65
?gured (l) to pass a ?rst portion of light travelling
along the ?rst path and to re?ect a second portion of
light travelling along the ?rst path so that the second
5,479,252
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20
portion of light travels along a second path not parallel
to the ?rst path, and (2) to pass a third portion of light
travelling to the beam splitter along the second path and
a white light source that emits white light;
a re?ective body for directing the white light toward the
object; and
to re?ect a fourth portion of light travelling to the beam
a camera for white-light imaging, the camera con?gured
to receive a portion of the white light re?ected from the
surface.
splitter along the second path so that the fourth portion
of light travels along the ?rst path;
a white light source that emits white light along the
second path to impinge the beam splitter and illuminate
27. The imaging system of claim 26, wherein the laser
light imaging and the white light imaging are performed
the surface;
a camera arranged to receive light travelling along the
second path; and
a ?lter arranged between the beam splitter and the camera,
the ?lter preventing laser light from impinging upon the
camera and allowing white light re?ected from the
surface and re?ected by the beam splitter to impinge
upon the camera.
26. An imaging system as in claim 22, further comprising:
10
simultaneously.
28. An imaging system as in claim 22, wherein the object
is enclosed within a region of the system and wherein air
within the region conforms to Class 1 cleanroom conditions.
29. An imaging system as in claim 22, wherein the beam
scanner is a resonant mechanical scanner.
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