U.S. patent application number 10/360512 was filed with the patent office on 2004-08-12 for system for detecting anomalies and/or features of a surface.
Invention is credited to Stokowski, Stanley E., Vaez-Iravani, Mehdi, Zhao, Guoheng.
Application Number | 20040156042 10/360512 |
Document ID | / |
Family ID | 27737542 |
Filed Date | 2004-08-12 |
United States Patent
Application |
20040156042 |
Kind Code |
A1 |
Vaez-Iravani, Mehdi ; et
al. |
August 12, 2004 |
System for detecting anomalies and/or features of a surface
Abstract
A cylindrical mirror or lens is used to focus an input
collimated beam of light onto a line on the surface to be
inspected, where the line is substantially in the plane of
incidence of the focused beam. An image of the beam is projected
onto an array of charge-coupled devices parallel to the line for
detecting anomalies and/or features of the surface, where the array
is outside the plane of incidence of the focused beam. For
inspecting surface with a pattern thereon, the light from the
surface is first passed through a spatial filter before it is
imaged onto the charge-coupled devices. The spatial filter includes
stripes of scattering regions that shift in synchronism with
relative motion between the beam and the surface to block Fourier
components from the pattern. The spatial filter may be replaced by
reflective strips that selectively reflects scattered radiation to
the detector, where the reflective strips also shifts in
synchronism with the relative motion.
Inventors: |
Vaez-Iravani, Mehdi; (Los
Gatos, CA) ; Zhao, Guoheng; (Milpitos, CA) ;
Stokowski, Stanley E.; (Danville, CA) |
Correspondence
Address: |
PARSONS HSUE & DE RUNTZ LLP
655 MONTGOMERY STREET
SUITE 1800
SAN FRANCISCO
CA
94111
US
|
Family ID: |
27737542 |
Appl. No.: |
10/360512 |
Filed: |
February 6, 2003 |
Current U.S.
Class: |
356/237.1 |
Current CPC
Class: |
G01N 21/94 20130101;
G01N 21/47 20130101; G01N 21/95623 20130101; G01N 2201/1045
20130101 |
Class at
Publication: |
356/237.1 |
International
Class: |
G01N 021/00 |
Claims
What is claimed is:
1. A method for detecting anomalies and/or features of a surface,
comprising: focusing a beam of radiation at an oblique incidence
angle to a line on the surface, a direction through the beam and
normal to the surface and said beam defining an incidence plane of
the beam, said line being substantially in the plane of incidence
of the beam; and imaging said line onto an array of detectors that
is substantially in the plane of incidence, each detector in the
array detecting light from a corresponding portion of the line.
2. The method of claim 1, said imaging being such that an image of
the line at said array is longer than the array.
3. The method of claim 1, wherein said focusing focuses the beam of
radiation substantially only along a direction normal to the plane
of incidence.
4. The method of claim 1, said focusing comprising passing a beam
of radiation through a cylindrical lens or to a cylindrical
mirror.
5. The method of claim 1, further comprising controlling a
cross-sectional dimension of the beam of radiation and said oblique
incidence angle in order to select length of the illuminated
line.
6. The method of claim 5, said controlling comprising expanding a
beam of radiation so that the beam has a desired diameter after
expansion.
7. The method of claim 1, wherein said imaging focuses the line
along an axis that is substantially normal to the line.
8. The method of claim 1, said imaging employing a variable
aperture, said method further comprising controlling the aperture
of the variable aperture in response to roughness of the
surface.
9. The method of claim 1, further comprising causing relative
motion between the surface and the beam so that the line scans the
surface to detect anomalies and/or a surface feature.
10. The method of claim 9, wherein said causing moves the surface
and leaves the beam and the array substantially stationary.
11. The method of claim 1, said oblique angle being in a range of
about 45 to 85 degrees from a normal direction to the surface.
12. The method of claim 1, further comprising performing time
delayed integration at the array of detectors.
13. The method of claim 1, wherein said focusing focuses the beam
to a thin focused line on the surface.
14. The method of claim 1, wherein said imaging causes an image of
said line to fall onto said array of detectors despite changes in
position of the surface.
15. An apparatus for detecting anomalies of a surface, comprising:
optics focusing a beam of radiation at an oblique incidence angle
to a line on the surface, a direction through the beam and normal
to the surface and said beam defining an incidence plane of the
beam, said line being substantially in the plane of incidence of
the beam; at least one array of detectors that is substantially in
the plane of incidence; and a system imaging said line onto the at
least one array of detectors, each detector in the at least one
array detecting light from a corresponding portion of the line.
16. The apparatus of claim 15, wherein an image of the line formed
by the system at the array is longer than the array.
17. The apparatus of claim 15, said optics focusing the beam
substantially only along a direction normal to the plane of
incidence.
18. The apparatus of claim 17, said optics comprising a cylindrical
lens or a cylindrical mirror.
19. The apparatus of claim 18, said optics comprising a cylindrical
lens that has a principal plane substantially parallel to the
surface.
20. The apparatus of claim 19, wherein said optics focuses an input
radiation beam directed at the optics; said input beam being
substantially normal to the surface and to the lens, said optics
further comprising a diffraction grating for redirecting radiation
from the lens towards the surface at an oblique angle to the
surface.
21. The apparatus of claim 19, wherein said optics focuses an input
radiation beam directed at the optics, said input beam being in a
direction oblique to the surface and to the principal plane of the
lens.
22. The apparatus of claim 18, said optics comprising a cylindrical
mirror that has two substantially straight edges, wherein a plane
defined by the two edges is substantially parallel to the
surface.
23. The apparatus of claim 15, further comprising means for causing
relative motion between the surface and the beam so that the line
scans the surface to detect anomalies and/or a surface feature.
24. The apparatus of claim 23, said at least one array of detectors
being substantially stationary relative to the beam when relative
motion is caused between the surface and the beam.
25. The apparatus of claim 15, said oblique angle being in a range
of about 45 to 85 degrees from a normal direction to the
surface.
26. The apparatus of claim 15, said system comprising lens means
having a Fourier plane, said apparatus further comprising a filter
and polarizer substantially in the Fourier plane.
27. A method for detecting anomalies associated with a surface with
a diffracting pattern, comprising: focusing a beam of
electromagnetic radiation to a line on the surface; causing
relative rotational motion between the surface and the beam;
causing scattered radiation from the line to interact with an array
of elongated strips that shift substantially in synchronism with
the rotational motion, so that diffraction from the pattern will
not reach a detector as the pattern rotates; and detecting
scattered radiation from the line by means of the detector.
28. The method of claim 27, wherein said causing comprises passing
scattered radiation from the line through a spatial filter
comprising an array of scattering or substantially opaque strips,
wherein said strips shift substantially in synchronism with the
rotational motion to block diffraction from the pattern as the
pattern rotates.
29. The method of claim 28, wherein said focusing focuses the
radiation at an oblique angle of incidence to the surface.
30. The method of claim 29, said beam and a line normal to the
surface defining a plane of incidence of the beam, wherein said
detecting detects radiation along a direction not in the plane of
incidence.
31. The method of claim 29, said beam and a line normal to the
surface defining a plane of incidence of the beam, wherein said
focusing focuses the radiation to the line on the surface so that
the line is substantially in the plane of incidence.
32. The method of claim 31, wherein said focusing focuses the
radiation to a focused line on the surface.
33. The method of claim 29, wherein said detecting detects
radiation along a direction substantially normal to the plane of
incidence.
34. The method of claim 29, wherein said detecting detects
radiation along a direction substantially in the plane of incidence
but away from a specular reflection of the beam.
35. The method of claim 27, further comprising shifting the strips
in a direction substantially parallel to the surface substantially
in synchronism with the rotational motion to block diffraction from
the pattern as the pattern rotates.
36. The method of claim 27, wherein said causing passes radiation
through a spatial filter comprising strips that are substantially
normal or parallel to the surface.
37. The method of claim 27, further comprising causing
translational motion between the surface and the beam, so that the
line scans a spiral path on the surface.
38. The method of claim 27, further comprising providing a signal
as a result of the detection indicative of said anomalies.
39. The method of claim 27, wherein said causing comprises
reflecting scattered radiation from the line to the detector,
wherein said strips shift substantially in synchronism with the
rotational motion so that diffraction from the pattern is not
reflected to the detector as the pattern rotates.
40. An apparatus for detecting anomalies associated with a surface
with a diffracting pattern, comprising: optics focusing a beam of
electromagnetic radiation to a line on the surface; an instrument
causing relative rotational motion between the surface and the
beam; a detector; and an array of elongated strips that interact
with scattered radiation from the line, wherein said strips shift
substantially in synchronism with the rotational motion so that
diffraction from the pattern does not reach the detector as the
pattern rotates; wherein the detector detects radiation from the
surface reaching the filter and provides a signal indicative of
said anomalies.
41. The apparatus of claim 40, said strips forming a spatial filter
that selectively passes scattered radiation from the line, said
strips comprising a radiation scattering material or substantially
opaque material.
42. The apparatus of claim 41, wherein said optics focuses the
radiation at an oblique angle of incidence to the surface.
43. The apparatus of claim 42, said beam and a line normal to the
surface defining a plane of incidence of the beam, wherein said
detector detects radiation along a direction not in the plane of
incidence.
44. The apparatus of claim 42, said beam and a line normal to the
surface defining a plane of incidence of the beam, wherein said
optics focuses the radiation to the line on the surface so that it
is substantially in the plane of incidence.
45. The apparatus of claim 42, wherein said detector detects
radiation along a direction substantially normal to the plane of
incidence.
46. The apparatus of claim 42, wherein said detector detects
radiation along a direction substantially in the plane of incidence
but away from a specular reflection of the beam.
47. The apparatus of claim 41, further comprising a device shifting
the strips in a direction substantially parallel to the surface
substantially in synchronism with the rotational motion to block
diffraction from the pattern as the pattern rotates.
48. The apparatus of claim 41, said filter comprising a layer of
liquid crystal material.
49. The apparatus of claim 41, said spatial filter comprising
stripes that are substantially normal or parallel to the
surface.
50. The apparatus of claim 41, wherein said instrument causes
translational motion between the surface and the beam, so that the
line scans a spiral path on the surface.
51. The apparatus of claim 41, wherein said array of elongated
strips reflects scattered radiation from the line to the detector,
wherein said strips shift substantially in synchronism with the
rotational motion so that diffraction from the pattern is not
reflected to the detector as the pattern rotates.
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates in general to surface inspection
systems, and in particular, to an improved system for detecting
anomalies and/or features of a surface.
[0002] The need to detect anomalies of a surface such as those on
the surface of a semiconductor wafer has been recognized since at
least the early 1980's. In the article "Automatic Microcircuit and
Wafer Inspection in Electronics Test," May 1981, pp. 60-70, for
example, Aaron D. Gara discloses a wafer inspection system for
detecting whether microcircuit chips are placed upside down or not
and for detecting flaws. In this system, a light beam from a laser
is passed through a beam expander and a cylindrical lens having a
rectangular aperture, where the lens focuses the beam to a narrow
line of laser light transverse to the incidence plane of the beam
to illuminate the wafer surface. It is stated in the article that
the smallest defect the system can reveal is less than 10 microns
wide.
[0003] The size of semiconductor devices fabricated on silicon
wafers has been continually reduced. The shrinking of semiconductor
devices to smaller and smaller sizes has imposed a much more
stringent requirement on the sensitivity of wafer inspection
instruments which are called upon to detect contaminant particles
and pattern defects as well as defects of the surfaces that are
small compared to the size of the semiconductor devices. At the
time of the filing of this application, design rule for devices of
down to 0.2 microns or below has been called for. At the same time,
it is desirable for wafer inspection systems to provide an adequate
throughput so that these systems can be used for in-line inspection
to detect wafer defects. One type of surface inspection system
employs an imaging device that illuminates a large area and images
of duplicate areas of surfaces, such as a target area and a
reference area used as a template, are compared to determine
differences therebetween. These differences may indicate surface
anomalies. Such system requires significant time to scan the entire
surface of a photomask or semiconductor wafer. For one example of
such system, see U.S. Pat. No. 4,579,455.
[0004] U.S. Pat. No. 4,898,471 to Stonestrom et al. illustrates
another approach. The area illuminated on a wafer surface by a
scanning beam is an ellipse which moves along a scan line called a
sweep. In one example, the ellipse has a width of 20 microns and a
length of 115 microns. Light scattered by anomalies of patterns in
such illuminated area is detected by photodetectors placed at
azimuthal angles in the range of 80 to 100.degree., where an
azimuthal angle of a photodetector is defined as the angle made by
the direction of light collected by the photodetector from the
illuminated area and the direction of the illumination beam when
viewed from the top. The signals detected by the photodetectors
from a region are used to construct templates. When the elliptical
spot is moved along the scan line to a neighboring region,
scattered light from structures within the spot is again detected
and the photodetector signal is then compared to the template to
ascertain the presence of contaminant particles or pattern defects.
While the scanning beam scans across the surface of the wafer, the
wafer is simultaneously moved by a mechanical stage in a direction
substantially perpendicular to the sweep direction. This operation
is repeated until the entire surface has been inspected.
[0005] While the system of Stonestrom et al. performs well for
inspecting wafers having semiconductor devices that are fabricated
with coarser resolution, with a continual shrinking of the size of
the devices fabricated, it is now desirable to provide an improved
inspection tool that can be used to detect very small anomalies
that can be difficult to detect using Stonestrom's system.
[0006] In the wafer inspection system where a light beam
illuminates a small area of the surface to be inspected, such as
those by Stonestrom et al. and Gara described above, the size of
the illuminated spot affects the sensitivity of the system. If the
spot is large relative to the size of the defects to be detected,
the system will have low sensitivity since the background or noise
signals may have significant amplitudes in relation to the
amplitudes of the signals indicating anomalies within the spot. In
order to detect smaller and smaller defects, it is, therefore,
desirable to reduce the size of the illuminated area on the wafer
surface.
[0007] However, as the size of the illuminated area is reduced,
throughput is usually also reduced. In addition, a smaller spot
size imposes a much more stringent requirement for alignment and
registration. As discussed above, in many wafer inspection systems,
it is common to perform a target image to a reference image
comparison for ascertaining the presence of anomalies. If the area
illuminated is not the intended target area but is shifted relative
to the target area, the comparison may yield false counts and may
become totally meaningless. Such shifting of the image relative to
the intended target area is known as misregistration.
[0008] Misregistration errors can be caused by misalignment of the
illumination optics due to many causes such as mechanical
vibrations, as well as by change in the position of the wafer such
as wafer warp or wafer tilt or other irregularities on the wafer
surface. For this reason, a wafer positioning system has been
proposed as in U.S. Pat. No. 5,530,550 to Nikoonahad et al. In this
patent, Nikoonahad et al. propose to use the specular reflection of
the scanning beam and a position sensitive detector for detecting
the change in height of the wafer and use such information to alter
the position of the wafer in order to compensate for a change in
height or tilting of the wafer surface.
[0009] While the above-described systems may be satisfactory for
some applications, they can be complicated and expensive for other
applications. It is, therefore, desirable to provide an improved
surface inspection system with improved sensitivity and performance
at a lower cost that can be used for a wider range of
applications.
[0010] In the inspection of samples with regular patterns thereon,
the scattering from such patterns may overwhelm signals from
anomalies of the sample. It is therefore desirable to provide an
improved surface inspection system with improved sensitivity and
performance for detecting anomalies of samples with patterns
thereon.
SUMMARY OF THE INVENTION
[0011] This application is related to U.S. patent application Ser.
No. 08/904,892 filed Aug. 1, 1997, which is referred to herein as
the "related application."
[0012] One aspect of the invention in the related application is
directed towards a method for detecting anomalies and/or features
of a surface, comprising focusing a beam of radiation at an oblique
incidence angle to illuminate a line on a surface, said beam and a
direction through the beam and normal to the surface defining an
incidence plane of the beam, said line being substantially in the
incidence plane of the beam; and imaging said line onto an array of
detectors, each detector in the array detecting light from a
corresponding portion of the line.
[0013] Another aspect of the invention in the related application
is directed towards a method for detecting anomalies of a surface
and/or a surface feature, comprising focusing a beam of radiation
at an oblique incidence angle to illuminate a line on the surface,
said beam and a direction through the beam and normal to the
surface defining an incidence plane of the beam; and imaging said
line onto an array of detectors outside of the incidence plane,
each detector in the array detecting light from a corresponding
portion of the line.
[0014] Yet another aspect of the invention in the related
application is directed towards an apparatus for detecting
anomalies of a surface comprising means for focusing a beam of
radiation at an oblique incidence angle to illuminate a line on the
surface, said beam and a direction through the beam and normal to
the surface defining an incidence plane of the beam, said line
being substantially in the incidence plane of the beam; at least
one array of detectors; and a system imaging said line onto the at
least one array of detectors, each detector in the at least one
array detecting light from a corresponding portion of the line.
[0015] One more aspect of the invention in the related application
is directed towards an apparatus for detecting anomalies of a
surface and/or a surface feature, comprising means for focusing a
beam of radiation at an oblique angle to illuminate a line on the
surface, said beam and a direction through the beam and normal to
the surface defining an incidence plane of the beam; at least one
array of detectors outside of the incidence plane; and a system
imaging said line onto the array of detectors, each detector in the
array detecting light from a corresponding portion of the line.
[0016] Yet another aspect of the invention in the related
application is directed to an apparatus for detecting anomalies
and/or a surface feature on a first and a second surface of an
object, comprising means for focusing a beam of radiation at an
oblique incidence angle to illuminate a line on the first surface,
said beam and a direction through the beam and normal to the first
surface defining an incidence plane of the beam, said line being
substantially in the plane of incidence of the beam; at least one
array of detectors; a system imaging said line onto the at least
one array of detectors, each detector in the at least one array
detecting light from a corresponding portion of the line; and means
for detecting anomalies and/or a surface feature of the second
surface.
[0017] One more aspect of the invention in the related application
is directed to an apparatus for detecting anomalies and/or a
surface feature on a first and a second surface of an object,
comprising means for focusing a beam of radiation at an oblique
angle to illuminate a line on the first surface, said beam and a
direction through the beam and normal to the first surface defining
an incidence plane of the beam; an array of detectors outside of
the plane of incidence; a system imaging said line onto the array
of detectors, each detector in the array detecting light from a
corresponding portion of the line; and means for detecting
anomalies and/or a surface feature of the second surface.
[0018] In the various aspects described above where the illuminated
line on the inspected surface is substantially in the plane of
incidence of the illumination beam, the detector may be outside the
plane of incidence in a double dark field configuration, or in the
plane of incidence but away from the specular reflection direction
of the beam in a single dark field configuration.
[0019] A surface inspection system with improved sensitivity and
performance can be achieved by focusing a beam of radiation to
illuminate a line on the surface of a sample and detecting
scattered radiation from the line. When the surface inspection
system is used to inspect samples with patterns such as arrays
(e.g. memory arrays) thereon, the scattered radiation from the
illuminated line is passed through a spatial filter prior to
detection. In order to inspect the entire surface, relative motion
is caused between the sample surface and the beam of radiation that
illuminates the line on the surface. If the relative motion between
the beam and the surface to inspect the whole surface is along
straight lines, then the blocking pattern of the filter may be set
after a learn cycle in order to shield the detector from the
scattering from the pattern on the surface when there is such
relative motion.
[0020] If the relative motion involves rotation between the
illumination beam and the surface, such as that achieved by
rotating the surface, then the scattering or diffraction pattern
from the sample surface moves relative to the beam and the filter
because of the rotational relative motion between the sample
surface and the beam. In such event, the above described process of
setting the filter blocking pattern after a learn cycle is
inadequate because of the relative motion of the scattering or
diffraction pattern relative to the filter. The spatial filter
preferably comprises an array of strips that scatter radiation or
are substantially opaque. In order to compensate for the relative
motion between the scattering or diffraction pattern and the
filter, the substantially opaque or scattering strips of the
spatial filter are also switched or otherwise shifted substantially
in synchronism with such relative motion in order to block
diffraction from the pattern on the sample surface as the pattern
moves. In general, the spatial filter may comprise any
configuration of areas (e.g. strips) that have radiation scattering
or transmission characteristics that are different from those of
the medium that separates the areas from one another; in such
event, the strips of the spatial filter are also switched or
otherwise shifted substantially in synchronism with relative motion
between the beam and the surface. These areas that are shifted
substantially in synchronism with relative motion of the scattering
or diffraction pattern may be in any shape designed to block
Fourier components or other scattering from the pattern.
[0021] Instead of using a spatial filter in the above embodiments
where relative motion between the sample surface and the
illumination beam is along straight or curved lines, substantially
the same effect can be achieved by reflecting the radiation
scattered by the surface by means of strips of reflective material
towards detectors, so that only scattered radiation that does not
contain the diffracted components from the pattern on the surface
is reflected to the detectors. Such and other variations are
possible. Where the relative motion is along a curved line, the
reflective strips may also be caused to shift (such as by
switching) with the moving diffracted components.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIG. 1 is a perspective view of a surface inspection system
to illustrate the invention in the related application.
[0023] FIG. 2 is a top view of the system of FIG. 1.
[0024] FIG. 3 is a perspective view of the illumination portion of
a surface inspection system to illustrate an alternative embodiment
of the invention in the related application.
[0025] FIG. 4 is a graphical plot of a point spread function useful
for illustrating the operation of the systems of FIGS. 1 and 3.
[0026] FIG. 5 is a schematic view of a parallel array of charged
coupled devices (CCD) useful for illustrating the invention in the
related application.
[0027] FIG. 6 is a schematic view of a light beam illuminating a
line on a surface and corresponding positions of detectors of an
array with respect to an imaging system along the line 6-6 in FIG.
2 to illustrate the operation of the system of FIGS. 1-3 in
response to height variation of the surface inspected.
[0028] FIG. 7 is a schematic view of the imaging optics, the CCD
detectors and a portion of the surface to be inspected of the
system of FIG. 1 taken along the line 7-7 in FIG. 2 to illustrate
the operation of the system of FIGS. 1-3 in response to height
variation of the surface to illustrate the invention in the related
application.
[0029] FIG. 8 is a schematic view of the collection and imaging
optics in the system of FIG. 1.
[0030] FIG. 9 is a perspective view of a portion of a wafer
inspection system employing a cylindrical mirror for illustrating
another alternative embodiment of the invention in the related
application.
[0031] FIG. 10 is a schematic view of a system for inspecting the
top and bottom surfaces of an object to illustrate another
embodiment of the invention in the related application.
[0032] FIG. 11 is a perspective view of the illumination portion of
a surface inspection system to illustrate still another alternative
embodiment of the invention in the related application.
[0033] FIG. 12 is a perspective view of a surface inspection system
employing a dynamically programmable spatial filter where the
illuminated line is in the plane of incidence of the illumination
beam to illustrate an embodiment of the invention.
[0034] FIG. 13 is a perspective view of a surface inspection system
employing a spatial filter and where the illumination beam is
substantially normal to the surface inspected to illustrate yet
another embodiment of the invention.
[0035] FIG. 14 is a perspective view of a surface inspection system
illuminating a line on the inspected surface where scattered light
from the line is detected by detectors in a single dark field
configuration to illustrate one more embodiment of the
invention.
[0036] FIG. 15 is a schematic view of the spatial filter of FIGS.
13-15 to illustrate a scheme for dynamically programming the
spatial filter.
[0037] For simplicity in description, identical components are
labeled by the same numerals in this application.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0038] FIG. 1 is a perspective view of a surface inspection system
to illustrate the preferred embodiment of the invention in the
related application. System 10 includes a cylindrical objective
such as a cylindrical lens 12 for focusing a preferably collimated
light beam 14 to a focused beam 16 for illuminating, on surface 18
to be inspected, an area in the shape of a line 20. Beam 14 and
therefore also focused beam 16 are directed at an oblique angle of
incidence to the surface 18. Different from the approach by Gara
described above, line 20 is substantially in the incidence plane or
plane of incidence of focused beam 16. In this context, the
incidence plane of beam 16 is defined by the common plane
containing beam 16 and a normal direction such as 22 to surface 18
and passing through beam 16. In order for the illuminated line 20
to be in the focal plane of lens 12, cylindrical lens 12 is
oriented so that its principal plane is substantially parallel to
surface 18. Image of the line is focused by an imaging subsystem 30
to an array of detectors, such as a linear array of CCDs 32. The
linear array 32 is preferably parallel to line 20. Preferably the
linear array 32 and the directions along which scattered radiation
is collected by array 32 are not in the plane of incidence of
focused beam 16, in a double dark field configuration as shown in
FIG. 1 and explained in more detail below. The incidence plane of
beam 16 is defined by the beam itself together with a direction
(such as 22) through the beam and normal to the surface. While beam
16 has a certain width, the plane of incidence is a plane that
would substantially pass through the midpoint of the width of beam
16.
[0039] In one embodiment particularly advantageous for detecting
small size anomalies, the imaging subsystem 30 has an optical axis
36 which is substantially normal to line 20 so that the center
portion of the linear CCD array 32 is in a plane substantially
normal to the incidence plane of beam 16. The optical axis 36 may
be oriented in any direction within such plane, including a
position directly above the line 20. In such event, array 32 would
also be directly above line 20 where the array is substantially
parallel to the surface 18. In other words, array 32 would be
substantially in the plane of incidence of beam 16 but away from
the specular reflection reflection of beam 16, in a single dark
field configuration. The position 32a of the detector array in a
single dark field configuration is shown in dotted lines in FIG. 1,
but the imaging lenses in the single dark field configuration have
been omitted to simplify the figure. If desired, another array 32'
shown in dotted line in FIG. 2 may be placed in a position
diametrically opposite to array 32, where array 32' has optical
axis 36' also substantially normal to line 20. The two arrays
together may be useful to detect 45 degree line patterns. The
detector array may also be placed at still other locations
different from those described above.
[0040] The imaging subsystem 30 projects an image of a portion of
the line 20 onto a corresponding detector in the CCD array 32 so
that each detector in the array detects light from a corresponding
portion of the line 20. The length of the line 20 is limited only
by the size of the collimated input beam 14 and the physical
aperture of lens or lens combination 12. In order to control the
length of line 20, an optional expander 34 shown in dotted lines
may be used for controlling the diameter of beam 14 so as to
control the length of line 20.
[0041] FIG. 3 is a perspective view of an illumination portion of a
wafer inspection system to illustrate an alternative embodiment of
the invention in the related application. To simplify the diagram,
the portion of the system for collecting and projecting an image of
the illuminated line onto a detector array has been omitted.
Instead of using a single symmetrical lens, the embodiment in FIG.
3 employs two cylindrical lenses 12' for tighter focusing, that is,
focusing to a thinner line. In FIG. 1, both the illumination and
collection portions of system 10 are stationary and surface 18 is
rotated about a spindle 50 which is also moved along direction 52
so that line 20 scans surface 18 in a spiral path to cover the
entire surface. As shown in FIG. 3, the surface 18' to be inspected
can also be moved by an XY stage 54 which moves the surface along
the X and Y directions in order for line 20 to scan the entire
surface. Again, the illumination and collection portions of system
10' of FIG. 3 remain stationary. This is advantageous since it
simplifies the optical alignment in the system, due to the fact
that there is substantially no relative motion between the
illumination portion and the collection portion of the system.
[0042] FIG. 4 is a graphical illustration of the point spread
function of focused line 20 along the focused direction along any
point of the line. As shown in FIG. 4, the point spread function of
line 20 is Gaussian in shape, such as one which is produced if an
488 nm argon laser is used. Line 20 may also exhibit a varying
point spread function along line 20 with a peak at the center of
line 20. In order to avoid the variation of intensity along the
line, it may be desirable to expand the beam by means of expander
34 to a longer length such as 10 mm and only use the center or
central portion of the line, such as the central 5 mm of the line,
so that power variation along the imaged portion of the line is
insignificant. By means of an appropriate aperture in the imaging
subsystem described below, it is possible to control the portion of
the line imaged onto the array.
[0043] FIG. 5 is a schematic view of the linear CCD array 32. As
shown in FIG. 5, the array 32 has dimension d in a direction
parallel to the line 20, and W is the illumination line width. In
other words, the image of line 20 as projected onto array 32 by
subsystem 30 has a width of W. The pixel size of the inspection
system 10 is determined by the scan pitch p and the pixel size of
the detectors in the array 32 in a direction parallel to line 20,
or d. In other words, the pixel size is dp. Thus, assuming that the
useful portion of the illumination line projected onto the CCD
array 32 has a length of 5 mm, and the illumination line width W is
10 microns and array 32 has 500 elements with d equal to 10 microns
and the scan line pitch is 5 microns, the effective pixel size on
the wafer is 5 microns.times.10 microns, assuming that the image of
the line at the array has the same length as the line. In practice,
to avoid aliasing, at least two or three samples are taken in each
direction (along line 20 and normal to it) per effective optical
spot size on the sample surface. Preferably, reasonably high
quality lenses such as quality camera lenses are used, such as ones
having 5 mm field of view, giving a 30.degree. collection
angle.
[0044] Lens 12 or lens 12' focus the illumination beam to a thin
focused line on surface 18. The width of line 20 is preferably
small, such as less than about 25 microns for improved
signal-to-noise ratio and higher resolution.
[0045] From the above, it is seen that system 10 has high
sensitivity, since the effective "pixel" size is 5.times.10
microns, which is much smaller than that of Stonestrom et al. At
the same time, due to the fact that the whole line of pixels on the
surface 18 are illuminated and detected at the same time instead of
a single illuminated spot as in Stonestrom et al., system 10 also
has acceptable throughput. As noted above, the length of line 20 is
limited only by the size of the collimated beam 14 and the physical
aperture of lens or lens combination 12. Thus, assuming that the
stage 54 has a stage speed of 10 microns per 0.1 millisecond, for a
line scan rate of 10 kHz, the surface can be scanned at a speed of
100 mm per second. For a line 20 of 5 mm, the wafer surface is then
scanned at a speed of 5 cm.sup.2/sec.
[0046] System 10 is also robust and tolerant of height variations
and tilt of surface 18 and 18'. This is illustrated in reference to
FIGS. 1, 2, 5-7. FIG. 6 is a cross-sectional view of a portion of
the surface 18 along the line 6-6 in FIG. 2, focused beam 16 and
two images of the array 32 when the surface 18 is at two different
heights. FIG. 7 is a cross-sectional view of the CCD array 32,
imaging subsystem 30 and two positions of a portion of the surface
18 to be inspected along the line 7-7 in FIG. 2.
[0047] In reference to FIGS. 1, 2 and 6, the imaging subsystem 30
will also project an image of the CCD array 32 onto surface 18
overlapping that of line 20. This is illustrated in FIG. 6. Thus,
if surface 18 is in the position 18A, then imaging subsystem 30
will project an image 32A of the detector array on surface 18A, as
shown in FIG. 6. But if the height of the surface is higher so that
the surface is at 18B instead, then the imaging subsystem will
project an image of the detector array at position 32B. The longer
dimension of beam 16 is such that it illuminates both images 32A
and 32B of the array.
[0048] From FIG. 6, it will be evident that the image of a
particular detector in the array will be projected on the same
portion of the surface 18 irrespective of the height of the
surface. Thus, for example, the imaging subsystem 30 will project
the first detector in the array 32 to position 32A(1) on surface
18A, but to the position 32B(1) on position 18B of the surface as
shown in FIG. 6. The two images are one on top of the other so that
there is no lateral shift between them. In the reverse imaging
direction, an image of the same portion of surface 18 and,
therefore, of line 20 will be focused to two different positions on
the array 32, but the two positions will also be shifted only in
the vertical direction but not laterally. Hence, if the detectors
cover both positions, then the variation in height between 18A, 18B
of the surface will have no effect on the detection by array 32 and
the system 10, 10' is tolerant of vertical height variations of the
surface inspected.
[0049] One way to ensure that the array 32 covers the images of
line 20 on surface 18 at both positions 18A, 18B is to choose
detectors in array 32 so that the dimension of the detectors in the
vertical direction is long enough to cover such change in position
of the surface, so that different positions of a portion of the
line 20 will be focused by subsystem 30 onto the detector and not
outside of it. In other words, if the vertical dimension of the
detector is chosen so that it is greater than the expected height
variation of the image of the line caused by height variation of
the wafer surface, the change in wafer height will not affect
detection. This is illustrated in more detail in FIG. 7.
[0050] As shown in FIG. 7, the pixel height (dimension normal to
optical axis and line 20) of array 32 is greater than the change in
position of the image of line 20 caused by a change in wafer
surface height, so that the imaging optics of subsystem 30 will
project the same portion of the surface and line on the wafer
surface onto the same detector. Alternatively, if the pixel height
of the CCD array 32 is smaller than the expected change in position
of image of line 20 due to height variation in the wafer surface,
multiple rows of CCDs may be employed arranged one on top of
another in a two-dimensional array so that the total height of the
number of rows in the vertical direction is greater than the
expected height variation of the line 20 image. If this total
height is greater than the expected movement of the image of the
line in the vertical direction, then such two-dimensional array
will be adequate for detecting the line despite height variations
of the wafer surface. The signals recorded by the detectors in the
same vertical column can be simply added to give the signal for a
corresponding portion of the line 20.
[0051] Even if the height or vertical dimension of array 32 is
smaller than the expected height variation of the wafer surface,
the imaging optics of subsystem 30 may be designed so that the
change in height or vertical dimension of the projected image of
line 20 onto the CCD array is within the height of the CCD array.
Such and other variations are within the scope of the invention.
Thus, in order for system 10 and 10' to be tolerant of wafer height
variation, the image of the line at the array 32 is longer than the
array, and the extent of the height variations of the image of the
line 20 on the detector array is such that the projected image
still falls on the detector array.
[0052] Where a two-dimensional array of detectors is employed in
array 32, time delayed integration may also be performed to improve
signal-to-noise or background ratio, where the shifting of the
signals between adjacent rows of detectors is synchronized with the
scanning of the line 20 across surface 18.
[0053] FIG. 8 is a schematic view illustrating in more detail the
imaging subsystem 30 of FIGS. 1 and 2. Subsystem 30 preferably
comprises two identical lenses: lens 102 for collecting light from
line 20 and to perform Fourier transform, and lens 104 for imaging
the line onto the array 32. The two lenses 102, 104 are preferably
identical to minimize aberration. A filter and polarizer may be
employed at position 106 where line 20, position 106 and array 32
appear at focal points of the two lenses 102, 104 each having a
focal length f. Arranged in this manner, subsystem 30 minimizes
aberration. As noted above, a variable aperture may also be applied
at a number of positions in subsystem 30 to control the portion of
the line 20 that is focused onto array 32 by controlling the size
of the aperture.
[0054] Instead of using a cylindrical lens 12 as shown in FIGS. 1
and 2, a cylindrical mirror may be used as shown in FIG. 9. In
order for line 20 to appear in the focal plane of cylindrical
mirror 112, the mirror should be oriented so that the plane 112'
defined by and connecting the edges 112a, 112b of the mirror is
substantially parallel to surface 18 inspected. In general, any
cylindrical objective that has the effect of focusing a beam 14
onto a focused line on surface 18 may be used, where the focusing
power is applied only in the direction substantially normal to the
incidence plane defined by focus beam 16 and a normal 22 to surface
18 through the beam.
[0055] An alternative method of generating a line focus on the
sample is to use a cylindrical lens in the convention way, i.e.
with its principal plane perpendicular to the propagation direction
of the light beam 14, and placing a diffraction grating 252
immediately following the lens. The grating period is such that
main diffraction angle matches the desired illumination angle
range. The lens and the grating are held parallel to each other,
and to the sample surface 18. The grating line structure (or
grooves) are perpendicular to the focused line direction. The
grating, therefore, will only have the effect of redirecting the
light along the desired incidence angle. Although a variety of
different grating types can be used, it is preferable to use a
holographic type grating for its enhanced efficiency.
[0056] By placing array 32 outside of the plane of incidence of
beam 16 in a double dark field configuration, signal-to-noise or
background ratio is improved over prior designs. A double dark
field collector configuration is one where the optical axis of the
collector in the subsystem is perpendicular to the optical axis of
illumination and the collector lies outside the incidence plane.
However, in some applications, it may be desirable to place the
array in the incidence plane. Preferably, beam 16 is at an angle in
the range of about 45 to 85 degrees from a normal direction to
surface 18. In addition to detection of anomalies, the invention
can also be used to detect other surface features such as
markers.
[0057] The invention as described above may be used to provide a
viable alternate mechanism to inspect rough films, patterned or
unpatterned semiconductor wafers and backsides of wafers, as well
as photomasks, reticles, liquid crystal displays or other flat
panel displays. The system of this invention is compact, has a
simple architecture, and provides a relatively low cost alternative
for inspecting patterned wafers. Furthermore, because of the low
cost of the system of this invention, it may also be advantageously
used in conjunction with another surface inspection system for
inspecting two different surfaces of an object, as illustrated in
FIG. 10. Thus, as shown in FIG. 10, a system 200 may include a
front side inspection system 202 for inspecting the front side 204a
of the semiconductor wafer 204, and a system 206 (which may be
similar to that in FIGS. 1, 2 or 3) for inspecting the backside
204b of the wafer. If, as in the invention described above, the
illumination and light collection portions of the system remain
stationary and the surface 204b is inspected by moving the surface,
the two systems 202, 206 may need to be synchronized. System 202
may include a system such as that described above in reference to
FIGS. 1-3, or may be one of many different kinds of anomaly and
surface feature inspection systems. All such variations are within
the scope of the invention.
[0058] When the above-described surface inspection system is used
for detecting anomalies of sample surfaces having patterns thereon,
such as arrays (e.g. semiconductor memory arrays, including DRAM,
SRAM etc.) thereon, the scattering from the patterns may overwhelm
signals from the anomalies. To prevent this from happening, the
scattered radiation from the illuminated line is passed through a
spatial filter prior to detection. In order to inspect the entire
surface, relative motion is caused between the sample surface and
the beam of radiation that illuminates the line on the surface. If
the relative motion between the beam and the surface to inspect the
whole surface is along straight lines, then the blocking pattern of
the filter may be set after a learn cycle in order to shield the
detector from the scattering from the pattern on the surface when
there is such relative motion. In such event, techniques such as
die-to-die comparison may be employed to further reduce the effects
of scattering from patterns. Where the pattern on sample surfaces
are not arrays, die-to-die comparison may still be employed to
reduce the effects of pattern scattering.
[0059] If the relative motion involves rotation between the
illumination beam and the surface, such as that achieved by
rotating the surface, then the scattering or diffraction pattern
from the sample surface moves relative to the beam and the filter
because of the rotational relative motion between the sample
surface and the beam. In such event, the above described process of
setting the filter blocking pattern after a learn cycle is
inadequate because of the relative motion of the scattering or
diffraction pattern relative to the filter. The spatial filter
comprises an array of scattering and transmitting strips. In order
to compensate for the relative motion between the scattering or
diffraction pattern and the filter, the scattering and transmitting
strips of the spatial filter are also switched substantially in
synchronism with such relative motion in order to block diffraction
from the pattern on the sample surface as the pattern moves. In one
embodiment, the spatial filter comprises an array of alternating
substantially opaque and transmitting strips; in such event, the
substantially opaque and transmitting strips of the spatial filter
are switched substantially in synchronism with relative rotational
motion between the beam and the surface.
[0060] The surface inspection system of FIG. 12 illustrates an
embodiment of the invention useful for inspecting sample surfaces
with patterns thereon, such as arrays (e.g. memory arrays) thereon,
where there is relative rotational motion between the beam and the
surface. Thus, as shown in FIG. 12, system 300 includes one or more
cylindrical lenses 12 which focus a collimated beam 14 onto a line
20 on the surface 18' of a semiconductor wafer in the same manner
as described above. As in the embodiment of FIG. 1, line 20 is
substantially in the plane of incidence of beam 14, where the plane
is defined by beam 14 and a line 22 which is normal to the surface
of wafer 18 where the line 22 intersects beam 14. As in the
embodiment of FIG. 1, a linear CCD camera 32 is used to detect
scattered light from the illuminated line 20 on surface 18' of a
wafer. The wafer is rotated about the spindle 50 which is also
moved along direction 52 so that the line 20 scans surface 18' is a
spiral path to cover the entire surface. Surface 18' of the wafer
has a regular pattern thereon, such as an array, which diffracts
the radiation in beam 14. The diffracted radiation takes on the
form of a two dimensional Fourier transform of the surface
pattern.
[0061] In the double dark-field configuration shown in FIG. 12, the
Fourier components are formed in the shape of narrow focused lines
in the back focal plane of the first collection lens 302.
Collection lens 302 collects the radiation scattered by the surface
18' within the illuminated line 20, and passes the radiation
through a polarizer 306 and a second lens 308 to the detector array
32. Spatial filter 304 comprises alternating scattering strips 304a
and transmitting strips 304b, where in one embodiment strips 304a
are substantially opaque. Thus, if a spatial filter 304 is placed
preferably at the back focal plane of lens 302, where the
substantially opaque narrow strips or stripes 304a are configured
to block the Fourier components from the pattern on surface 18',
the intensity of the Fourier components detected by camera 32 would
be much reduced or eliminated so that a camera 32 will be able to
detect scattered radiation from anomalies on surface 18' without
being overwhelmed by the Fourier components.
[0062] However, when surface 18' is rotated by spindle 50 along
direction of arrow 50', the Fourier components diffracted by the
pattern on the surface would also rotate. This means that even
though when stationary, spatial filter 304 is effective in blocking
the Fourier components diffracted by the pattern on surface 18',
when the surface is rotated, the fixed substantially opaque and
transmitting strips of the spatial filter would no longer be
effective in blocking the Fourier components. Another aspect of the
invention is based on the recognition that, by effectively
switching the substantially opaque and transmitting striped regions
304a of the filter in synchronism with the rotation of surface 18'
along direction 304' that matches direction 50' of spindle 50,
filter 304 would be effective in blocking the Fourier components
from reaching the detector 32 as the surface is rotated. This is
possible even though motion of the stripes 304a and 304b may be
along a straight line 304' within the collection aperture defined
by the collection lens 302. One example would demonstrate the
feasibility of the scheme. Preferably line 304' is substantially
parallel to the surface 18'.
[0063] It is assumed that the point spread function of line 20 is
Gaussian, so that the width of line 20 may be defined by the
distance between the points across its width where the intensity
falls below 1/e.sup.2 of the peak intensity, e being the natural
number. If the line width of line 20 is 10 microns and its length
20 millimeters, the line may be used to scan a 200 millimeter wafer
in six turns. If each wafer is scanned in 60 seconds, then this
means each turn of the wafer is performed in 10 seconds. It is
assumed that the azimuthal collection angle of lens 302 is
32.degree., and that filter 304 has 16 stripes so that each stripe
corresponds to 2.degree. in azimuth. The time it takes for the
wafer to rotate by 2.degree. is about {fraction (1/18)} seconds.
This speed is slow enough for filter 304 to be a liquid
crystal-type filter. In other words, as the wafer rotates, filter
304 is adjusted every {fraction (1/18)} second, in synchrony with
the wafer rotation, to reject the Fourier components.
[0064] What emerges from the other side of filter 304 is the
scattered radiation from surface 18', where the scattered radiation
comprises surface scattering due to the roughness, as well as
scattering due to any defects or anomalies, including particles.
The scattered radiation emerging from the filter is passed through
a polarizer 306 and imaged by a second lens 308 onto a camera CCD
array 32 preferably placed at the image plane of the optical
collection system that includes lenses 302 and 308. Where line 20
has a line width of 10 microns and a length of 20 millimeters, and
array 32 has 1024 elements, each pixel on the wafer will be about
10.times.20 microns in size. The CCD camera 32 may be chosen with
an appropriate operation frequency to accommodate the required
rotation rate of surface 18'. If line 20 is 10 microns by 20
millimeters and CCD array 32 has 1024 elements, camera 32 may use a
10 MHz CCD, operating at a line rate of 10 kHz. The data
acquisition rate of array 32 changes with the position of line 20
on surface 18'. As line 20 approaches the center of the wafer, the
data acquisition rate would decrease. Thus, it may be desirable for
camera 32 to have a variable clock rate to account for the
variation in the scan speed as line 20 approaches the center of the
wafer. If desirable, the modulation speed for the filter can be
reduced by either increasing the length of the line, or increasing
the width of the line, which results in a slower rotation rate.
Another way to reduce the modulation rate of the filter 304 is to
reduce throughput, or the speed of rotation or translation of
spindle 50. Where there is rotational motion between the beam 14,
16 and surface 18', it may be difficult to record the orientation
of dies on the surface of a semiconductor wafer inspected to
perform die-to-die comparison. In such event, techniques such as
wafer-to-wafer comparison may be employed to further reduce the
effects of scattering from patterns. In such scheme, the exact
signal levels and coordinates of events on an entire wafer are
stored, and the signal levels compared to those for the same pixels
of another wafer in a wafer-to-wafer comparison.
[0065] If system 300 is used for inspecting unpatterned wafers,
filter 304 may be operated so that all of the stripes are
transparent. Alternatively, for such applications, filter 304 may
be removed. The motion of the stripes 304a of filter 304 is
controlled by a power supply 310 which is controlled in turn by
computer 312. Computer 312 also controls the line rate of camera 32
and of the rotation and translation of spindle 50. Thus, computer
312 controls all three operations so that the substantially opaque
stripes 304a of filter 304 move in synchronism with motion of the
Fourier components as surface 18' is rotated, and so that camera 32
is operated at a high enough frequency to collect data.
[0066] In system 300 of FIG. 12, the collection subsystem comprises
lenses 302, 308, filter 304, polarizer 306 and camera 32; these
elements are placed to collect radiation scattered in directions
outside of the plane of incidence of beam 14 in a double dark-field
configuration. The invention may also be used in other types of
configurations as illustrated in FIGS. 13 and 14. In FIG. 13, the
collimated illumination beam 14' is supplied in a direction
substantially normal to surface 18' as shown in FIG. 13. Other than
such difference, system 320 of FIG. 13 operates in substantially
the same manner as system 300 of FIG. 12.
[0067] FIG. 14 illustrates another embodiment 330 of the invention.
System 330 differs from system 300 of FIG. 12 in that the radiation
collection subsystem is placed to collect radiation scattered in
directions substantially normal to surface 18', and in that line 20
is not in the plane of incidence of beam 14". Except for such
differences, system 330 operates in substantially the same manner
as system 300 of FIG. 12. Aside from the different configurations
shown in FIGS. 12-14, still other configurations are possible. For
example, instead of locating the radiation collection subsystem in
FIG. 12 to collect radiation scattered in directions outside of the
plane of incidence of beam 14, it is possible to re-orient the
collection subsystem of FIG. 12 to a position substantially as
shown in FIG. 14, in a single dark field configuration, to collect
radiation scattered within the plane of incidence but away from a
specular reflection of the beam. The detector array is then at
position 32a as shown in dotted lines in FIG. 12, where the strips
304a and 304b (not shown) would then be substantially parallel to
the wafer surface 18'. The detector array and strips 304a, 304b may
also be placed at locations other than those described above.
[0068] Preferably, filter 304 is oriented so that the stripes 304a
are substantially normal to surface 18', as illustrated in the
solid line position of the filter in the double dark-field
configuration. The output of CCD camera 32 is supplied to computer
312 for data capture and analysis in order to determine whether
there is an anomaly on surface 18'. Beams 14 and 14" are preferably
supplied at oblique angles to surface 18'.
[0069] FIG. 15 is a schematic view of filter 304 and power supply
310 to illustrate a scheme for controlling the movement or shifting
of the stripes 304a, 304b. Filter 304 includes a reference
electrode 304(1) and an array of elongated electrodes 304(2) where
each of the electrodes in the array can be individually addressed
by power supply 410. The reference electrode 304(1) is separated
from the array 304(2) by a liquid crystal material 304(3) (not
shown). Therefore, by applying a suitable voltage to the reference
electrode 304(1) and suitable voltages to the electrodes in array
304(2) by means of power supply 310 through electrical conductors
(the connectors to the individual electrodes in array 304(2) are
not shown to simplify the figure). This will allow one to control
the voltage across an elongated region of the liquid crystal
material corresponding to each of the elongated electrodes in array
304(2) and the reference electrode, and whether such region of the
liquid crystal material is transparent or opaque to radiation.
These elongated regions of the liquid crystal material define the
stripes 304a. Thus, by controlling the voltages across the array
304(2) and electrode 304(1), the transmittance of each of the
stripes 304a, 304b can be controlled individually as a function of
time. In this manner, the movement of the stripes 304a, 304b, or
the switching of these stripes along direction 304'can be
synchronized with the rotational motion 50' of surface 18'.
Obviously, other embodiments for moving or shifting strips or
stripes 304a, 304b are possible, such as where scattering,
substantially opaque or reflective strips are mechanically moved in
synchronism with the rotational motion 50' of surface 18'. Such and
other variations are within the scope of the invention.
[0070] Instead of using a spatial filter in the above embodiments
where relative motion between the sample surface and the
illumination beam is along straight or curved lines, substantially
the same effect can be achieved by reflecting the radiation
scattered by the surface by means of strips of reflective material
towards detectors, so that only scattered radiation that does not
contain the diffracted components from the pattern on the surface
is reflected to the detectors. The reflective strips would be
placed at a location away from the location shown in FIG. 12 to one
(shown in dotted lines 304" in FIG. 12) that can selectively
reflect scattered radiation from line 20 to the detector.
Appropriate optics may be used in a manner known to those skilled
in the art to relay the scattered radiation from the surface 18' to
the reflective strips and from the strips to the array of
detectors. Such and other variations are possible. Where the
relative motion is along a curved line, the reflective strips may
also be caused to shift (such as by switching) with the moving
diffracted components.
[0071] While the invention has been described by reference to
various embodiments, it will be understood that modification
changes may be made without departing from the scope of the
invention which is to be defined only by the appended claims or
their equivalents. All references referred to herein are
incorporated by reference in their entireties.
* * * * *