U.S. patent application number 10/601698 was filed with the patent office on 2004-05-06 for wafer inspection system for distinguishing pits and particles.
This patent application is currently assigned to ADE Optical Systems Corporation. Invention is credited to Clementi, Lee D., Fossey, Michael E., Stover, John C..
Application Number | 20040085533 10/601698 |
Document ID | / |
Family ID | 27364022 |
Filed Date | 2004-05-06 |
United States Patent
Application |
20040085533 |
Kind Code |
A1 |
Fossey, Michael E. ; et
al. |
May 6, 2004 |
Wafer inspection system for distinguishing pits and particles
Abstract
A surface inspection system and method is provided which detects
defects such as particles or pits on the surface of a workpiece,
such as a silicon wafer, and also distinguishes between pit defects
and particle defects. The surface inspection system comprises an
inspection station for receiving a workpiece and a scanner
positioned and arranged to scan a surface of the workpiece at the
inspection station. The scanner includes a light source arranged to
project a beam of P-polarized light and a scanner positioned to
scan the P-polarized light beam across the surface of the
workpiece. The system further provides or detecting differences in
the angular distribution of the light scattered from the workpiece
and for distinguishing particle defects from pit defects based upon
these differences.
Inventors: |
Fossey, Michael E.; (Newbury
Park, CA) ; Stover, John C.; (Charlotte, NC) ;
Clementi, Lee D.; (Lake Wylie, SC) |
Correspondence
Address: |
FISH & RICHARDSON P.C.
1425 K STREET, N.W.
11TH FLOOR
WASHINGTON
DC
20005-3500
US
|
Assignee: |
ADE Optical Systems
Corporation
|
Family ID: |
27364022 |
Appl. No.: |
10/601698 |
Filed: |
June 24, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10601698 |
Jun 24, 2003 |
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10301677 |
Nov 22, 2002 |
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10301677 |
Nov 22, 2002 |
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09906062 |
Jul 17, 2001 |
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6509965 |
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09906062 |
Jul 17, 2001 |
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09624502 |
Jul 24, 2000 |
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6292259 |
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09624502 |
Jul 24, 2000 |
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08958230 |
Oct 27, 1997 |
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6118525 |
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08958230 |
Oct 27, 1997 |
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08399962 |
Mar 6, 1995 |
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5712701 |
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60032103 |
Dec 4, 1996 |
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Current U.S.
Class: |
356/239.7 |
Current CPC
Class: |
G01N 21/9501 20130101;
H01L 22/12 20130101; G01N 2021/945 20130101 |
Class at
Publication: |
356/239.7 |
International
Class: |
G01N 021/00 |
Claims
That which is claimed:
1. A surface inspection system for distinguishing between particle
defects and pit defects on a surface of a workpiece, the surface
inspection system comprising: an inspection station for receiving a
workpiece; a scanner positioned and arranged to scan a surface of
the workpiece at said inspection station, said scanner including a
light source arranged to project a beam of P-polarized light and
means positioned to scan the P-polarized light beam across the
surface of the workpiece; and means for detecting differences in
the angular distribution of the light scattered from the workpiece
and for distinguishing particle defects from pit defects based upon
said differences.
2. A surface inspection system as defined in claim 1, wherein said
means for detecting differences in the angular distribution of the
scattered light includes means for comparing the amount of light
scattered in a direction substantially perpendicular from the
surface of the workpiece to the amount of light backscattered from
the surface of the workpiece.
3. A surface inspection system as defined in claim 1, wherein said
means for detecting differences in the angular distribution of the
scattered light includes means for identifying a dip in the
intensity distribution of the scattered light.
4. A surface inspection system as defined in claim 1, wherein said
means for detecting differences in the angular distribution of the
scattered light comprises a plurality of collectors positioned and
arranged for collecting light at different angles relative to the
surface of the workpiece, said collectors each including a
photodetector for generating signals in response to the collected
light, and means for comparing the signals from photodetectors
located at said different angles.
5. A surface inspection system as defined in claim 4, wherein said
plurality of collectors includes a first collector positioned and
arranged to collect light components scattered generally
perpendicular from the surface of the workpiece, and a second
collector positioned and arranged to collect light components
scattered angularly from the surface of the workpiece.
6. A surface inspection system as defined in claim 5, wherein said
scanner is positioned and arranged to direct the light beam onto
the surface of the workpiece at a predetermined angle of incidence
other than perpendicular, and said second collector is positioned
and arranged to collect backscattered light components.
7. A surface inspection system as defined in claim 6, wherein said
plurality of collectors additionally includes a third collector
positioned and arranged to collect light components forwardly
scattered from the surface of the workpiece.
8. A surface inspection system as defined in claim 6, wherein said
scanner is positioned and arranged to direct the light beam onto
the surface of the workpiece at an angle of incidence of at least
-50.degree. from perpendicular to the workpiece surface.
9. A surface inspection system as defined in claim 5, wherein said
first collector is arranged to collect light components scattered
over an angle of approximately .+-.20.degree. from perpendicular to
the workpiece surface.
10. A surface inspection system as defined in claim 1, additionally
including means to form a first map identifying the locations of
pit defects on the workpiece surface and a second map identifying
the locations of particle defection the workpiece surface.
11. A surface inspection system as defined in claim 10, including a
video display operatively associated with said means to form said
first and second maps for displaying a visual representation of
said first and second maps.
12. A surface inspection system for distinguishing between particle
defects and pit defects on a surface of a workpiece, the surface
inspection system comprising: an inspection station for receiving a
workpiece; a scanner positioned and arranged to scan a surface of a
workpiece at said inspection station, said scanner including a
light source arranged to project a beam of P-polarized light at an
angle of incidence at least 50 degrees rearward of perpendicular to
the workpiece surface, and means positioned to receive the light
beam and to scan the light beam along a predetermined scan path
across a surface of the workpiece; a detector arranged for
detecting light scattered from the surface of a workpiece at said
inspection station, said detector including a center channel
collector positioned and arranged to collect light components
scattered substantially perpendicular from the surface of the
workpiece, and a back channel collector positioned and arranged to
collect light components backscattered from the surface of the
workpiece, said collectors each including photodetectors for
generating electrical signals in response to the collected light;
and a comparator responsive to electrical signals from the
photodetectors of said center channel collector and said back
channel collector for detecting differences in the angular
distribution of the scattered P-polarized light from the
workpiece.
13. A surface inspection system as defined in claim 12, wherein
said inspection station comprises: a transporter arranged to
translationally transport a workpiece along a material path; and a
rotator associated with said transporter and arranged to rotate
workpiece during translational travel along the material path.
14. A surface inspection system as defined in claim 13, wherein
said scanner is positioned and arranged to scan a surface of a
workpiece during rotational and translational travel along the
material path to produce a spiral scan pattern over the surface of
the workpiece.
15. A surface inspection system as defined in claim 12, wherein
said comparator includes a first comparer for identifying a defect
as a particle if the ratio of the intensity of the center channel
photodetector signal to the back channel photodetector signal is
less than a predetermined amount.
16. A surface inspection system as defined in claim 15, wherein
said detector additionally includes a forward channel collector
positioned and arranged to collect light components forwardly
scattered from the surface of the workpiece, said forward channel
collector including a photodetector for generating an electrical
signal in response to the collected light, and wherein said
comparator includes a second comparer for identifying a defect as a
pit if the ratio of the intensity of the center channel
photodetector signal to the forward channel photodetector signal is
more than a predetermined amount.
17. A surface inspection method for distinguishing between particle
defects and pit defects on a surface of a workpiece, said method
comprising: scanning a beam of P-polarized light across the surface
of the workpiece; collecting light scattered from the surface of
the workpiece; and detecting differences in the angular
distribution of the light scattered from the workpiece and
distinguishing particle defects from pit defects based upon said
differences.
18. A method as defined by claim 17, wherein said step of detecting
differences in the angular distribution of the scattered light
comprises comparing the amount of light scattered in a direction
generally perpendicular from the surface of the workpiece to the
amount of the scattered backwardly from the surface of the
workpiece.
19. A method as defined by claim 17, wherein said step of detecting
differences in the angular distribution of the scattered light
comprises identifying a dip in the intensity distribution of the
scattered light.
20. A method as defined by claim 17, wherein said step of detecting
differences in the angular distribution of the scattered light
includes comparing the intensity of scattered light collected at
various angles relative to the surface of the workpiece.
21. A method as defined by claim 20, wherein said step of detecting
differences in the angular distribution of the scattered light
includes comparing the intensity of light components scattered
substantially perpendicular from the surface of the workpiece to
the intensity of the light components scattered angularly from the
surface of the workpiece.
22. A method as defined by claim 17, additionally including forming
a first map identifying the locations of pit defects on the
workpiece surface.
23. A method as defined by claim 22, including forming a second map
identifying the locations of particle defects on the workpiece
surface.
24. A method as defined in claim 23, including displaying said
first and second maps on a video display.
25. A method as defined by claim 21, wherein said step of scanning
a beam across the surface of the workpiece comprises projecting the
beam of light onto the workpiece at an angle of incidence of at
least 50 degrees from perpendicular to the workpiece.
26. A surface inspection method for distinguishing between particle
defects and pit defects on a surface of a workpiece, said method
comprising: receiving a workpiece at an inspection station;
scanning a surface of a workpiece at the inspection station with a
beam of P-polarized light at an angle of incidence of at least 50
degrees rearward of perpendicular to the workpiece surface;
collecting light scattered from the surface of a workpiece at the
inspection station with a first collector positioned and arranged
to collect light components scattered generally perpendicular from
the surface of the workpiece, and with a second collector
positioned and arranged to collect light components backscattered
from the surface of the workpiece; converting the collected light
components from the first collector and the second collector into
respective electrical signals; and analyzing the signals to
distinguish particle defects from pit defects.
27. A surface inspection method as defined in claim 26, wherein
said analyzing step comprises identifying a defect as a particle if
the ratio of the intensity of the first collector signal to the
second collector signal is less than a predetermined amount.
28. A surface inspection method as defined claim 27, wherein said
collecting step additionally includes collecting light scattered
from the surface of a workpiece at the inspection station with a
third collector positioned and arranged to collect light components
scattered forwardly from the surface of the workpiece and
converting the collected light into an electrical signal, and
wherein said analyzing step includes identifying a defect as a pit
if the ratio of the intensity of the first collector signal to the
third collector signal is more than a predetermined amount.
29. A surface inspection method for distinguishing between particle
defects and pit defects on a surface of a workpiece, said method
comprising: receiving a workpiece at an inspection station;
translationally transporting the workpiece along a material path at
the inspection station; rotating the workpiece during translational
travel along the material path; scanning a beam of P-polarized
light across the surface of the rotating and translationally
transported workpiece at an angle of incidence of at least 50
degrees from perpendicular to the workpiece; collecting scattered
light at a first location about perpendicular to the surface of the
workpiece and at a second location rearwardly therefrom; detecting
differences in the angular distribution of the scattered light from
the workpiece; and analyzing the detected differences in angular
distribution to distinguish particle defects from pit defects.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application claims priority from provisional
application Serial No. 60/032,103 filed Dec. 4, 1996.
FIELD AND BACKGROUND OF THE INVENTION
[0002] The present invention relates to surface inspection systems
and methods and, more particularly, to the inspection of articles
or workpieces, such as silicon wafers, to detect the presence of
defects such as particles or pits on the surface and to distinguish
therebetween.
[0003] Surface inspection systems are commonly used for the
inspection of articles or workpieces such as silicon wafers to
detect the presence of defects on the wafer surface. When the
inspection indicates a large number of defects, the wafer may be
sent back for recleaning. If the defects are particles or other
debris on the wafer surface, the recleaning is successful. However,
if the defects are pits or "COPS" (crystal originated pits) in the
wafer surface, they are not removed by recleaning. Because such
surface inspection systems fail to distinguish between sit defects
and particle defects, the wafer is typically sent back for
recleaning regardless of whether the defects are pits or particles.
Because these defects may be pits, recleaning the wafer may result
in nothing more than a waste of time and resources. It would be
advantageous to be able to distinguish pits in the surface of the
wafer from particles located thereon.
SUMMARY OF THE INVENTION
[0004] The present invention provides a surface inspection system
and method which not only detects defects such as particles or pits
on the surface of a workpiece, such as a silicon wafer, but also
distinguishes between pit defects and particle defects. This makes
it possible to easily ascertain whether the workpiece requires
recleaning to remove particle defects, or whether other measures
must be taken.
[0005] In a broad aspect, the surface inspection system comprises
an inspection station for receiving a workpiece and a scanner
positioned and arranged to scan a surface of the workpiece at said
inspection station. The scanner includes a light source arranged to
project a beam of P-polarized light and a scanner positioned to
scan the P-polarized light beam across the surface of the
workpiece. The system further provides for detecting differences in
the angular distribution of the light scattered from the workpiece
and for distinguishing particle defects from pit defects based upon
these differences. The differences in the angular distribution of
the scattered light may, for example, be detected by comparing the
amount of light scattered in a direction substantially
perpendicular from the surface of the workpiece to the amount of
light backscattered from the surface of the workpiece. The
detection of differences in the angular distribution of the
scattered light may also, for example, involve identifying a dip in
the intensity distribution of the scattered light.
[0006] The scanner preferably scans across the surface of the
workpiece along a relatively narrow scan path during rotational and
translational travel of the workpiece. More specifically, the
system preferably has a transporter arranged for transporting the
workpiece along a material path and a rotator associated with the
transporter and arranged for rotating the workpiece during
translational travel along the material path. The scanner is
positioned and arranged or scanning a surface of a workpiece during
rotational and translational travel along the material path so that
the entire surface of he workpiece is raster scanned in a spiral
pattern. The scanner includes either a P-polarized light source or
a light source coupled with a P-polarized filter positionally
aligned with the light source.
[0007] A collector also is arranged for collecting light reflected
and scattered from the surface of the workpiece during rotational
and translational travel along the material path. The collector
includes a dark channel detector positioned for detecting light
which is scattered from the surface of a workpiece. The dark
channel detector includes a plurality of collectors positioned and
arranged for collecting light at different angles relative to the
surface of the workpiece. Each collector includes a photodetector
for generating electrical signals in response to the collected
light. The electrical signals from photodetectors located at the
different angles are compared to determine the differences in
angular distribution of the scattered light.
[0008] The plurality of collectors preferably includes a forward
channel collector arranged to collect light components scattered
forwardly from the surface of the workpiece at a relatively small
angle with respect to the specular reflection from the workpiece, a
center channel collector positioned adjacent to the forward channel
collector and arranged to collect light components scattered
substantially normal from the surface of the workpiece at a
relatively medium angle, and a back channel collector positioned
adjacent to the center channel collector and arranged to collect
light components scattered backwardly from the surface of the
workpiece at a relatively large angle.
[0009] When the scanned light beam contacts a defect, such as a pit
or a particle, light is scattered from the surface and is collected
by the collectors. The intensity of the scattered light, and the
time of its detection during the scan, provide information about
the size and location or the defect on the surface of the
workpiece. Furthermore, the nature of the defect, i.e. whether it
is a pit or a particle, can be ascertained by detecting differences
in the angular distribution of the light scattered from the
workpiece.
[0010] For example, if the defect is a pit, the amount of light
scattered and detected by the center channel collector is typically
greater than that detected by the back channel collector.
Alternatively, if the defect is a particle, the amount of the light
detected by the center channel collector is typically less than
that detected by the back channel collector and/or the forward
channel collector. The dark channel collector system provides the
surface inspection system of the present invention with high
sensitivity to more readily identify, classify, and/or provide a
topography of the condition of the surface of an article or a
workpiece, including defects such as particles, pits and the like,
in and on the surface of a workpiece.
[0011] According to one specific embodiment of the invention, a
P-polarized light beam is directed along a predetermined relatively
narrow scan path and at a relatively low angle of incidence with
respect to the surface of the workpiece. The method preferably also
includes imparting a rotational and translational movement of the
workpiece during the narrow scan so that the narrow scan path
traverses the entire surface of the workpiece along a spiral
path.
[0012] The surface inspection system and method of the present
invention advantageously distinguish pits in the surface of the
wafer from particles on the surface of the wafer and therefore
determine whether cleaning or some other course of action, e.g.,
altering the conditions of manufacture and storage, can be used to
cure the defects. In addition, the surface inspection system and
method provide high spatial resolution, a small field of view at
the object plane which, in turn, provides improved edge detection
performance, improved repeatability in the inspection process and
reduces interference signals caused by scatter from air
molecules.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] Other advantages will appear as the description proceeds
when taken in connection with the accompanying drawings, in
which:
[0014] FIG. 1 is a perspective view of a surface inspection system
according to the present invention.
[0015] FIG. 2 illustrates a transporter of a surface inspection
system according to the present invention arranged for rotatively
and translationally transporting a workpiece, such as a wafer,
along a material path.
[0016] FIG. 3 schematically illustrates a side elevational view of
a surface inspection system according to the present invention.
[0017] FIG. 3A is a fragmentary view of a light channel detector of
a surface inspection system according to the present invention.
[0018] FIG. 4 schematically illustrates a side elevational view of
an optical scanning system of a surface inspection system according
to the present invention.
[0019] FIG. 5 schematically illustrates rotational and
translational travel of a wafer through an inspection area
according to the present invention.
[0020] FIG. 6 schematically illistrates a collector of a surface
inspection system having segmented optics for collecting light
scattered from a surface of a wafer according to the present
invention.
[0021] FIG. 7 schematically illistrates a system controller of a
surface inspection system according to the present invention.
[0022] FIG. 8 illustrates a comparison between using S-polarized
and P-polarized light at normal and non-normal angles of incidence
to distinguish pits and particles in and on the surface of a
wafer.
[0023] FIG. 9 illustrates the use of P-polarized light at a
non-normal angle of incidence in detecting pits and several types
of particles in and on the surface of a wafer.
[0024] FIG. 10 illustrates a comparison between using P-polarized
and S-polarized light in detecting a particle on the surface of a
wafer and provides both experimental and modeled results.
[0025] FIG. 11 illustrates the use of P-polarized light at a
non-normal angle of incidence in detecting scattered light for pits
of various diameters in the surface of a wafer.
[0026] FIG. 12 illustrates the use of P-polarized light in
detecting pits of various diameters in a wafer.
[0027] FIG. 13 illustrates the angular distribution pattern of
relative small COPS and particles using P-polarized light at a
non-normal angle of incidence.
[0028] FIG. 14 is an illustration similar to FIG. 13 showing the
angular distribution pattern of medium size COPS and particles.
[0029] FIG. 15 is an illustration similar to FIG. 13 showing the
angular distribution pattern of larger size COPS and particles.
[0030] FIG. 16 is a flowchart illustrating the application of an
algorithm for distinguishing between COPS and particles.
[0031] FIGS. 17 and 18 are graphs illustrating how the constants
for the algorithm of FIG. 16 may be derived.
[0032] FIG. 19 is a particle man of a clean wafer.
[0033] FIG. 20 is a COP map of a clean wafer.
[0034] FIG. 21 is a particle map of the wafer of FIG. 19 after
particle defects of known size have been deposited thereon.
[0035] FIG. 22 is a COP map,of the wafer of FIG. 21 with the
particle deposition present, but not detected as COP defects.
DESCRIPTION OF ILLUSTRATED EMBODIMENT
[0036] The present invention will be described more fully
hereinafter with reference to the accompanying drawings in which
specific embodiments of the invention are shown. This invention
may, however, be embodied in many different forms and should not be
construed as limited to the illustrated embodiments set forth
herein; rather, these illustrated embodiments are provided so that
this disclosure will be thorough and complete and will fully convey
the scope of the invention to those skilled in the art. Like
numbers refer to like elements throughout.
[0037] FIG. 1 is a perspective view of a surface inspection system
20 for detecting defects such as particles, pits and the like on a
surface of a workpiece W or article, such as a silicon wafer.
Portions of the system 20 are broken away for purposes of clarity
and shown by phantom lines to illustrate various elements of the
surface inspection system 20. The surface inspection system 20 is
suitably used for inspecting the surface of unpatterned wafers W,
both with and without deposited films. The system 20 preferably
includes means for translationally transporting a workpiece W along
a material path P, means associated with he translational
transporting means or rotating the workpiece W as it travels along
the material path P, means for scanning the surface S of the
workpiece W during rotative and translating travel along the
material path P, and means for collecting light reflected and
scattered from the surface S of the workpiece W.
[0038] As illustrated in FIG. 1, the surface inspection system 20
is arranged as a workstation including a worktable 21. Positioned
on the worktable 21 is a generally closed and substantially light
proof housing 22, a video display 23, a keyboard 25, and a mouse
26. A cabinet 27 is suspended from the worktable for carrying a
system controller 50. Adjacent the cabinet 27 is a shelf unit 28
for carrying a printer 29 and associated printing paper 29a. The
housing 22 has been partially broken away to better illustrate the
inspecting arrangement of the present invention. The inspection of
the wafer W preferably is conducted in an inspection zone Z on an
inspection table 31. A robotic wafer handling device 32 is located
adjacent the inspection station 20 to load and unload wafers W from
a cassette 33 onto the table 31. The cassette 33 holds a number of
wafers W and is loaded into the cabinet 27 through a door (not
shown). The handling of the wafers W inside the housing 22 is done
automatically without contact by human hands to avoid contamination
or smudges.
[0039] As best illustrated in FIGS. 1-3, the surface inspection
system 20 preferably includes means for translationally
transporting a workpiece W along a material path P. The means for
transporting a workpiece W is illustrated as a transporter 40
arranged translationally transport a work-piece W along a material
path P and preferably through an inspection zone or area Z. The
translational transporter 40, as illustrated, preferably includes a
gear 42, a motor 41 including a shaft 41a arranged for rotating the
gear 42, and guides 36, 37 having teeth formed integral therewith.
The motor 41 and gear 42 mounted on the motor shaft 41a form a
chuck for the system 50. The motor 41 of the chuck is preferably
mounted to a stage member 43 having a plurality of flanges 43a
extending upwardly therefrom which receives the workpiece W, i.e.,
silicon wafer, thereon along edges of the workpiece W as
illustrated. This mounting technique for the workpiece W reduces
smudges or other surface problems which may be associated with
positioning the lower surface of the workpiece so as to abuttingly
contact an upper surface of the stage member 43. The stage member
43 preferably is translationally transported along stage guide
members 38, 39 secured to an underside thereof. Other translational
and/or rotating means such as a piston and cylinder configuration
mounted to the stage member and a motor for rotating the stage
member as understood by those skilled in the art may also be used
according to the invention.
[0040] Also, means for rotating a workpiece W, illustrated as a
rotator 45, is associated with the transporter 40 and arranged to
rotate a workpiece W during translational travel along the material
path P. The rotator 45 as illustrated preferably includes a motor
46 mounted to an underside of the stage member or providing
rotation of the wafer mounted thereon at a predetermined speed. The
transporter 40 and the rotator 45 preferably are synchronized and
arranged with a scanner 80 so as to form a spiral-shaped narrow
angle scan (.alpha.) across the surfaces of the workpiece during
rotational and translational travel along the material path P.
[0041] As illustrated in FIGS. 1 and 3-5, a scanner 80 is
positioned and arranged to scan a surfaces of a workpiece W during
rotational and translational travel along the material path P. It
will also be understood, however, by those skilled in the art that
the scanner 80 may be arranged for rotational and/or translational
movement while the workpiece W is stationary, or translationally or
rotatively moved. In addition, other material paths P may be used,
e.g., neither the workpiece W nor the scanner 80 may be
translationally moved and the workpiece tested in only a rotational
path. Accordingly, the present invention includes a P-polarized
light source 81 or a light source coupled with a P-polarized filter
positionally aligned with the light source to generate a
P-polarized light beam B therefrom, means for receiving the light
source and scanning a surface S of a workpiece W, i.e., a mirror
82, lenses 84, 86, deflector 85, and means for imparting a
rotational and translational scan of the workpiece W, i.e., the
transporter 40 and the rotator 45.
[0042] The scanner 80 preferably includes a light source 81, i.e.,
laser, arranged to either generate a P-polarized light beam B
therefrom or coupled with a P-polarized filter positionally aligned
with the light source. The P-polarized light preferably has a spot
size which includes a full width, half-maximum of less than 0.1
millimeters. The scanner also includes means positioned to receive
the light beam B and arranged for scanning the light beam B along a
relatively narrow scan path (.alpha.) across a surface S of the
workpiece W as the workpiece W rotationally and translationally
travels along the material path P. The light source 81 is
preferably a visible-light laser with a relatively short
wavelength, such as Argon-Ion or solid state, as understood by
those skilled in t he art. The laser 81 is also preferably the
combination of a laser with external optics as understood by those
skilled in the art. The laser 81 preferably has a bean diameter of
about 0.6 millimeters ("mm").
[0043] The scanning means preferably includes a deflector 85, as
illustrated, positioned to receive the light beam B and arranged to
deflect the light beam B along a relatively narrow scan path
(.alpha.). The deflector 85 is preferably an acousto-optical (AO)
deflector as illustrated (or a mechanical deflector), and the
relatively narrow scan path (.alpha.) is preferably no greater than
0.1 radians and, more particularly, in the range of 0.025-0.040
radians. The scan path .alpha. preferably directionally corresponds
to the path P of translational travel and, as best illustrated in
FIG. 4, preferably is in a generally parallel direction therewith
as illustrated by the arrows. The deflection to is accomplished by
exciting a crystal with high frequency sound waves, for example,
which interact with the incident light wave in such a way to shift
the light beam B and thereby change the angle of propagation. It
will be understood that various frequencies of the crystal will
responsively cause the light passing therethrough to be deflected
at correspondingly various angles of propagation. If the frequency
of the sound waves is swept in a sawtooth pattern, the laser beam B
is scanned through an angle (.alpha.) proportional to the
frequency. The AO deflector 85 preferably provides a constant
scanning speed which, in turn, provides a consistent or a
predetermined time response for particles or defects detected from
an article surface. Although the present invention is described
with reference to an AO deflector 85, other means for providing
narrow angle scans as understood by those skilled in the art, such
as a galvanometer, a piezoelectric scanner, a resonant scanner, a
rotating mirror, a scanning head, other electronic scanners, or the
like, may also be used according to the present invention.
[0044] Also, a beam expander 82 is preferably positioned between
the laser source 81 and the deflector 85 to expand the light bean B
prior to entering the acousto-optical deflector 85. The beam
expander 82 preferably provides means for more fully filling the
active aperture of the deflector 85 to best utilize the scan angle
of the deflector 85.
[0045] The scanner 80 also preferably includes means positionally
aligned with the deflector 85 and arranged or directing the light
beam from the narrow scan path (.alpha.) toward a surface S of a
workpiece W at a relatively low angle of incidence (.theta..sub.i)
(relative to the workpiece surface) as the workpiece W rotatively
and translationally travels along the material path P. Although a
low angle of incidence (.theta..sub.i) is preferred, the angle of
incidence (.theta..sub.i) may be any angle other than normal to the
workpiece W to provide the advantages of the present invention. The
angle of incidence is preferably greater than 45 degrees from
normal to the article surface, i.e., less than 45 degrees from the
surface of the workpiece W and, more particularly, is preferably in
the range of 65-85 degrees from normal to the article surface.
[0046] The directing means is illustrated as a mirror 82 and a
plurality of optical lenses 84, 86 arranged to direct the light
beam B from the laser 81 toward the surface S of the workpiece W to
be inspected. As the light beam B travels from the AO deflector 85,
the beam B passes through a cylindrical lens 84 which preferably
angularly orients the light beam B for a linear scan of the surface
of the article daring translational and rotational movement of the
article through the inspection zone. A stop Member 87 is
positionally aligned with the cylindrical lens 84 positioned
closely adjacent the AO deflector 85 to stop the relatively small
portion of light which is not linearly oriented for the scan of the
surface of the workpiece W. The optical lens 86 positioned after
the cylindrical lens 84 is a focusing or f-theta lens, as
understood by those skilled in the art, arranged for focusing the
light beam on the surface of the workpiece W.
[0047] The scanner 80 according to the present invention preferably
scans the beam of light B in a radial direction with rotating
motion and linear, lateral, or translational motion (Y) to
implement a spiral scan pattern as best illustrated in FIG. 3.
Nevertheless, any other material path P for the workpiece W may
also be used to provide the advantages of the invention.
[0048] As best illustrated in FIGS. 1, 3, 3A, and 5-7, means for
collecting light from the surface of a workpiece is preferably a
collector 100 having a light channel detector 110 arranged for
detecting light specularly reflected from the surface S of a
workpiece W and a dark channel detector 120 positioned adjacent the
light channel detector 110 for detecting light scattered from the
surface S of a workpiece W. The light channel detector 110 may be a
PMT or a photodiode, but preferably, as understood by those skilled
in the art, is a quadrant-cell device, i.e., detector, arranged for
X--Y coordinate positioning detection so that deviation in the path
of reflected light, i.e., during detection of a defect or particle,
may be determined. Such quadrant-cell detectors are manufactured by
Advanced Photonix, Inc., formerly Silicon Detector Corp., of
Camarillo, Calif. Although a particular configuration is
illustrated, it will be understood that various other rectangular
or multiple cell, i.e., bi-cell, configurations may also be used
according to the present invention.
[0049] The dark channel detector 120 preferably includes a
plurality of collectors 121, 123, 125 positioned closely adjacent
each other and arranged for collecting components of the scattered
light at different respective predetermined angles from the surface
S of the workpiece W. The plurality of collectors 121, 123, 125 of
the dark channel detector 120 form segmented optics having at least
two collectors positioned adjacent each other. The plurality of
collectors 121, 123, 125 as illustrated will be understood by those
skilled in the art to be compound lenses, and other lens
arrangements may also be used according to the present invention.
The plurality of collectors 121, 123, 125 respectively include a
forward channel collector 121 arranged to collect light components
scattered forwardly from the surface S of the workpiece W at a
relatively small angle a, a center channel collector 123 positioned
closely adjacent the forward channel collector 121 and arranged to
collect light components scattered substantially normal from the
surface S of the workpiece W at a relatively medium angle b, and a
back channel collector 125 positioned closely adjacent the center
channel collector 123 and arranged to collect light components
scattered backwardly from the surface S of the workpiece W at a
relatively large angle c. The dark channel detector 120 further
includes a forward channel detector 122, a center channel detector
124, and a back channel detector 126 each respectively positioned
in optical communication with a corresponding collector 121, 123,
125, and means electrically connected to the forward, center and
back channel detectors 122, 124, 126 and responsive to electrical
signals from said detectors for determining the presence of a
particle on the surface S of a workpiece W. The determining means
of the collector is preferably electronic signal discrimination
circuitry 150, such as illustrated (see FIGS. 3 and 7) and
understood by these skilled in the art, which receives signals
representative of collected light from the light channel detector
110 and the dark channel detector 120.
[0050] As best illustrated in FIGS. 1, 3, and 6, the relative
respective angles a, b, c of the plurality of collectors 121, 123,
125 are preferably determined with respect to the angle of
reflection (.theta..sub.r) of light from the surface S of the
workpiece W and with respect to forward a, backward c, and
substantially normal b light component scattering which occurs
relative to the angle of incidence .theta..sub.i of the scan. For
example, if the angle of incidence .theta..sub.i is relatively low
with respect to the surface plane (high with respect to nor,-,.al,
e.g., 15.degree. from horizontal or -75.degree. from normal, then
the forward scattering or small angle a is preferably about
+22.degree. to +67.degree., the substantially normal scattering or
medium angle is about -25.degree. to +20.degree., and the backward
scattering or large angle is about -72.degree. to -27.degree.. In
addition, the advantages of the present invention have been
realized, for example, where the angle of incidence .theta..sub.i
is 25.degree. from horizontal or -65.degree. from normal with
P-polarized visible light having a wavelength of 488 nm, the back
channel collector is centered at -38.degree., and the center
channel collector is centered at +10.degree.. When a particle or
defect is detected, for example, the forward channel collector 121
is positioned to receive and collect forward scattering a, the
center channel collector 123 is positioned to receive and collect
substantially normal scattering b, and the back channel collector
125 is positioned to receive and collect back scattering c from the
surface of the workpiece with respect to the detected particle or
defect, or the like. In the direction generally perpendicular to
the plane of incidence, approximately 73.degree. of total angle is
captured in the above example. This is about 0.64 steridians of
solid angle per segment or a total of about 1.92 steridians which
is a substantial improvement over previous known detectors.
[0051] As best illustrated in the perspective view of FIG. 1 and
the schematic view of FIG. 7, the surface inspection system 20
preferably is computer controlled. The system controller 50
operates the inspection system 20 under the supervision and
direction of a human operator, stores and retrieves data generated
by the system 20, and performs data analysis preferably responsive
to predetermined commands. The scanner assembly portion 90
illustrates cooperates with the scanner 80 and includes a chuck
detector 91 which transmits a position to a servo-amplifier 91. The
relative position of the article being inspected is communicated to
the system 50 via motors 41, 46 and encoders 93 mounted thereto.
The position data is transmitted to the AO scan control 73 which
preferably forms a portion of the system electronics chassis 70 and
which responsively drives the AO deflector 85 via a AO scan driver
95.
[0052] The system electronics chassis 70 includes a system power
supply 71 and receives signals from the dark channel detectors 120
and the light channel detector 110 respectively representative of
the scattered and the specularly reflected light. As understood by
those skilled in the art, these data signals are conventionally
electrically communicated in an analog format to analog front end
electronics 75 and are converted to digital format by digital front
end electronics 74 or the like. The digital front end electronics
74 also cooperates with the AO scan control 73, the system bus
interface 72, and the differential interface 69, i.e., differential
bus, of the personal computer ("PC") chassis 60. The system bus
interface 72 also communicates with a laser power supply 51 of the
surface inspection system 50.
[0053] The PC chassis 60 includes a PC power supply 61 arranged for
supplying power to the PC. The PC chassis 60 also has a motion
controller 64 which responsively communicates with the servo
amplifier 92 of the scanner assembly 90 and a system control
computer 65, i.e., microprocessor, or controller. The system
control computer 65 preferably electrically communicates with a
wafer handler 52, for responsively sending and receiving
predetermined command signals for mounting and handling the article
or wafer being inspected as set forth above. The system control
computer 65 also preferably communicates with a hard disk drive 68,
a display adapter 67 arranged to communicate with the display, and
an ethernet interface 66 arranged for network or other system 50
communication. An image processor 64 electrically communicates with
the differential interface 69 and the system control computer 65
for processing the image of the surface of the inspected article
and/or defects, flaws, undulations, or particles thereon. The
surface inspection system 50 as illustrated in FIG. 7, and as
understood by those skilled in the art, preferably is formed of a
combination of software and hardware which forms these various
components, or combinations thereof, of the system 50.
[0054] As illustrated in FIGS. 1-7, methods of inspecting a surface
S of an article or workpiece W for defects are also provided
according to the present invention. A method of inspecting a
surface S of a workpiece W preferably includes rotatively and
translationally transporting a workpiece W along a material path P
and scanning a relatively narrow scan path .alpha. of light across
a surface of the workpiece W as the workpiece W travels along the
material path P. The step or rotatively and translationally
transporting a workpiece along a material path preferably is
synchronized with the step of scanning a surface of the workpiece
so as to impart a substantially spiral-shaped scan of the surface
of the workpiece. Light specularly reflected from and light
scattered from the surface S of the workpiece W preferably are
separately collected.
[0055] The light which is scattered from the workpiece surface is
collected as separate light components at different angles. For
example, light components scattered substantially normal from the
surface S of the workpiece W and light components scattered
backwardly from the surface S of the workpiece W are separately
collected and compared to thereby ascertain differences in the
angular distribution of the scattered light. Light scattered from
the surface S of the workpiece W is separately collected by a
plurality of collectors 121, 123, 125 at a plurality of
predetermined scattering angles a, b, c. Preferably, the collectors
are positioned to collect forwardly scattered light components,
backwardly scattered light components, and light components
scattered in a direction substantially perpendicular to the surface
of the workpiece. Light detected by the various collectors
signifies a defect in or on the surface S of the workpiece w.
[0056] In order to determine whether the defect is a particle
defect or a pit, differences in the angular distribution of the
light scattered from the workpiece are detected. This is achieved
by comparing the amount of light collected by one of the collectors
to the amount of light collected by one or more of the other
collectors. The light detected by the detectors 122, 124 and 126,
particularly the center channel detector 124 and the back channel
detector 126 can be used to distinguish particles located on the
workpiece surface from pits located in the workpiece surface when
P-polarized light is used in the scanner specifically, when the
defect is a pit in the workpiece surface, P-polarized light
scattered from the workpiece surface forms a pattern in which the
amount of light scattered to the center channel collector 124 is
greater than the amount of light scattered to the back channel
collector 126. This has been found to be particularly the case with
small pits, i.e., pits having a diameter of no more than about 330
nm. In contrast, when the defect is a particle on the workpiece
surface, P-polarized light scattered from the wafer surface forms a
pattern in which the amount of light scattered to the center
channel collector 124 is less than the light scattered to the back
channel collector 126. FIGS. 8-15 show examples of the scattered
light distributions for pits and particles of various sizes when
P-polarized light is used. FIG. 8 is a comparison between scatter
diagrams for 90 nm tungsten particles on the surface of the
workpiece and 180 nm pits in the surface of the workpiece. As seen
from the two scatter diagrams on the left side of FIG. 8, the use
of either S-polarized light and the use of a normal or
perpendicular angle of incidence, i.e. , .theta..sub.i=0.degree.,
with either S or P-polarized light does not provide an effective
method of distinguishing the particles from the pits in the surface
of the workpiece. The angular distribution of the light scattered
from the workpiece is substantially similar for pit defects and for
particle defects. Likewise, as seen from the scatter diagram in the
upper right quadrant, when S-polarized light is used at a
non-normal angle of incidence, for example
.theta..sub.i=-70.degree., there is relatively little difference in
the shape of the scatter curves for pits and for particles.
However, when P-polarized light is used, as seen from the diagram
in the lower right quadrant, the particles scatter light in such a
way that a dip is detected in the region approximately
perpendicular to the workpiece surface. The pits create a
distinctly different angular distribution pattern, by which pits
can we distinguished from particles.
[0057] FIG. 9 illustrates the angular distribution patterns
obtained from various particle materials. When P-polarized light is
used at an angle of incidence, .theta..sub.i=-70.degree., the
particles may be distinguished by a characteristic dip in the
region approximate the angle normal the workpiece surface
(0.degree.) thus allowing the presence of particles on the surface
of the workpiece to be distinguished from the presence of pits
within the workpiece. Specifically, the 120 nm psl articles, the 90
nm silicon particles, the 80 nm tungsten particles, and the 75 nm
aluminum particles all exhibit a characteristic dip in the vicinity
of the direction normal or perpendicular to the surface of the
workpiece (0.degree.). The specific minimum point varies for each
particle, but all are generally within a region covering .+-. about
25.degree. from the 0 or perpendicular direction. The scattering
pattern from the pit does not exhibit a dip.
[0058] FIG. 10 compares the angular distributions for modeled
results and experimental results using a 0.1 micron psl sphere with
a laser beam at a wavelength of 488 nm and a -75.degree. angle of
incidence using both P-polarized light and S-polarized light. As
shown, the P-polarized light produces a characteristic dip in the
vicinity of 0.degree.. No such dip occurs using S-polarized
light.
[0059] FIG. 11 illustrates modeled scatter for pits of various
diameters. As shown in FIG. 11, when P-polarized light is used at
an angle of incidence, .theta..sub.i=-70.degree., the amount of
backwardly scattered light is greater than the amount of forwardly
scattered light for small pits. This is particularly the case where
the diameter of the pit is no more than about 300 nm.
[0060] FIG. 12 illustrates the angular distribution of light
scattered bad larger pits, i.e., pits having a diameter of more
than about 430 nm, located in the surface of the workpiece.
[0061] FIGS. 13, 14 and 15 illustrate representative angular
distribution patterns for small COPS, medium COPS and large COPS
respectively, versus particles using P-polarized light at an angle
of incidence of -70.degree.. In FIG. 13, it will be seen that a 120
nm cut exhibits a convex shared distribution pattern, with the
amount of light backscattered at angles ranging from -20 to -60
being higher than the light forwardly scattered at angles of +20
and above. Particles of various compositions with sizes of 90 nm
and below all exhibit a characteristic concave distribution
pattern, with a "dip" in the vicinity of 0.degree.. For the larger
sized particles, e.g. 90 mn. psl, the, intensity of forwardly
scattered light is greater than the backscattered light.
[0062] As seen from FIG. 14, the angular distribution pattern for a
somewhat larger 155 nm COP is generally similar to the 120 nm cup
of FIG. 13, with the amount of backscattered light at angles of
from -20 to -80 being greater than the amount of forwardly
scattered light. The smaller sized particles, e.g. 91 nm and
smaller consistently exhibit a concave distribution pattern with a
"dip" at or near 0.degree., and with the amount of forwardly
scattered light being greater than the amount of backscattered
light. The larger particles (120 nm psl) show a markedly greater
difference between the amount of forwardly scattered light and
backscattered light. This trend is also seen in FIG. 15 with
particles as large as 200 nm.
[0063] From these characteristic angular distribution patterns, it
is possible to distinguish COPS from particles. In particular, if
the ratio of the intensity of the signal from the center channel
detector 124 to the signal from the back channel detector 125 is
less than a predetermined amount, the defect may be classified as a
particle. If the ratio of the intensity of the center channel
detector 124 signal to the forward channel 122 detector signal is
more than a predetermined amount, the defect may be classified as a
pit. FIG. 16 illustrates one suitable algorithm for analyzing the
information from the detectors to distinguish particles from COPS.
If the ratio of the center channel indicated size C to the back
channel indicated size B is less than the predetermined constant,
in this instance 1.14, then the defect is classified as a particle.
Stated otherwise, a signal event B representing the back channel
indicated size and a signal event C representing the is center
channel indicated size are directed to a comparator where the value
of C is compared to the value of B times a predetermined constant,
in this instance 1.14. If C is not greater than 1.14 times B, then
the signal event is classified as a particle. If C is greater than
1.14 times B, then C is compared to a value F representing the
forward channel indicated size. If C is greater than a
predetermined constant (in this instance 1.36) times the value of
F, then the signal event is classified as a COP. If not, the event
is classified as a particle.
[0064] The application of this algorithm as applied to COPS is
graphically illustrated in FIG. 17. The application of this
algorithm to particles, in this instance aluminum particles, is
graphically illustrated in FIG. 18. FIGS. 17 and 18 illustrate how
modeled data or experimental data for particles or COPS of various
sizes can be used to derive constants for use in the type of
algorithm shown in FIG. 16. It should be apparent to those of skill
in the art from this illustration that the present invention is not
limited to the particular algorithm described herein, and that
other approaches and other specific algorithms may be used to
process the data obtained from the various detectors and to
distinguish between pits and particles in accordance with the
present invention.
[0065] During the scanning of a wafer, as signal events are thus
classified into particle defects and pit defects, the resulting
data may be stored in memory in a suitable format, such as a raster
format, to define a "map" of the particles or pits on the surface
of the wafer. In addition, the intensity values of the signal
events, representing the indicated sizes of the particles or pits,
may be stored to provide a histogram illustrating the size
classifications of the defects. This information may be
communicated to the user as a visual representation of the wafer on
a video display. FIG. 19, for example, illustrates a video display
which presents a particle map of a clean wafer, together with a
histogram showing the distribution of particle sizes. FIG. 20 shows
a map of the COPS or pits for the same wafer, and a size histogram
for the pits. FIGS. 21 and 22 illustrate the sensitivity and
selectivity of the apparatus and method of this invention. The same
wafer which was used to produce the particle map of FIG. 19 was
"seeded" with particle defects of known size in three regions on
the wafer. FIG. 21 is a particle map of that wafer, and the three
regions of seeded particles are clearly evident. FIG. 22 is a COP
map of that same wafer. By comparing FIG. 20 (before seeding) with
FIG. 22 (after seeding) it is evident that the COP map and the
histogram are substantially unaffected by the heavy concentrations
of seeded particles on the wafer.
[0066] In the drawings and specification, there have been disclosed
typical preferred embodiments of the invention, and, although
specific terms are employed, these terms are used in a descriptive
sense only and not for purposes of limitation. The invention has
been described in considerable detail with specific reference to
various illustrated embodiments. It will be apparent, however, that
various modifications and changes can be made within the spirit and
scope of the invention as described in the foregoing specification
and defined in the appended claims.
* * * * *