Particle detection system with reflective line-to-spot collector

The surface inspection system has a scanning head for scanning the laser beam along a predetermined scan line across the surface of an article. A collector receives the light reflected from the article surface along the scan line. The collector has a first mirror position for receiving light reflected from the article surface, a second mirror oriented with respect to the first mirror to receive light reflected from the first mirror, and the first and second mirrors being configured and oriented so as to concentrate the reflected light from a line into a spot. A photodetector is positioned for receiving the thus formed spot of light. The method of inspecting the surface of an article includes the steps of scanning a laser beam along a predetermined scan line across the surface of the article, collectively receiving the light reflected from the article surface along the scan line with a plurality of mirrors so as to concentrate the reflected light from a line into a spot.

FIELD OF THE INVENTION 
This invention relates to surface inspection systems, and more particularly 
to the inspection of articles, such as silicon wafers, for flaws or 
defects in the surface of the article. 
BACKGROUND OF THE INVENTION 
In the process of manufacturing silicon or other semiconductor microchips, 
light is generally directed through a reticle mask to etch circuits into a 
silicon wafer. The presence of dirt, dust, smudges or other foreign matter 
on the surfaces of the reticle mask or the silicon wafer is highly 
undesirable and adversely affects the resulting circuits. As a result, the 
reticles and the silicon wafers are necessarily inspected before use. One 
common inspection technique is for a human inspector to visually examine 
each surface under intense light and magnification. Debris that is smaller 
than can be visually detected by the human eye, however, impairs the 
resulting microchips. 
Laser inspection systems therefore have been developed for inspecting the 
surface of silicon wafers to accurately detect small particles. In these 
conventional laser inspection systems, light is both specularly reflected 
and scattered from the surface of an article. The specularly reflected 
light and the scattered light are both indicative of the presence of 
particles or flaws on the surface of the article. The light specularly 
reflected from the surface and the light scattered from the surface are 
collected and separately relayed to photodetectors such as a 
photomultiplier tube ("PMT") or a charge coupled device ("CCD"). 
Several laser inspection systems have been developed which provide various 
types collectors, such as fiber optic bundles, spherical or parabolic 
mirrors, elongated lenses, and light pipes, for collecting the light and 
separately relaying the light to photodetectors. Examples of such systems 
may be seen in U.S. Pat. No. 4,875,780 by Moran et al. entitled "Method 
and Apparatus for Inspecting Reticles"; U.S. Pat. No. 4,795,911 by Kohno 
et al. entitled "Surface Examining Apparatus For Detecting The Presence of 
Foreign Particles on the Surface"; U.S. Pat. No. 4,630,276 by Moran 
entitled "Compact Laser Scanning System"; U.S. Pat. No. 4,601,576 by 
Galbraith entitled "Light Collector For Optical Contaminant And Flaw 
Detector"; U.S. Pat. No. 4,378,159 by Galbraith entitled "Scanning 
Contaminant And Defect Detector"; U.S. Pat. No. 4,376,583 by Alford et al. 
entitled "Surface Inspection Scanning System"; and U.S. Pat. No. 4,360,275 
by Louderback entitled "Device for Measurement of Optical Scattering". The 
collectors of these systems, however, are often bulky and awkward for 
installation into commercial laser inspection machines and are often 
inefficient in collecting portions of the light. 
Thus, there is a need for a particle detection system which compactly and 
efficiently collects the light specularly reflected and scattered from the 
surface of an article and focuses the light into a photodetector. 
SUMMARY OF THE INVENTION 
The present invention provides a particle detection system having a 
relatively compact and efficient line-to-spot collector for collecting the 
specularly reflected or the scattered light from the surface of an article 
and reflecting the light into a photodetector. The line-to-spot collector 
has a plurality of mirrors positioned for receiving the light either 
specularly reflected or scattered from the surface of an article. A first 
mirror is curved and a second mirror is flat, and the curvature of the 
first mirror causes the light which is reflected along a scan line from 
the surface of the article to be focused into a predetermined spot. The 
configuration and orientation of the mirrors are such that the light 
reflected or scattered into the line-to-spot collector is compactly and 
efficiently collected so that the amount of light lost in the transfer 
process to the photodetector is minimized. 
More particularly, the surface inspection system has a scanning mirror for 
scanning a laser beam along a predetermined scan line across the surface 
of an article. A collector receives the light reflected from the article 
surface along the scan line. The collector has a first mirror positioned 
for receiving light reflected from the article surface and a second mirror 
oriented with respect to the first mirror to receive light reflected from 
the first mirror. The first and second mirrors are configured and oriented 
so as to concentrate the reflected light from a line into a spot. A 
photodetector is positioned for receiving the thus formed spot of light. 
By folding the light path, such that it is reflected from each of the 
reflective surfaces of the mirrors a plurality of times, it is possible to 
significantly increase the overall effective length of travel of the 
specularly reflected or scattered light received by the collector 
positioned between the scanned article and the photodetector, i.e., the 
focal length, within a very compact apparatus. The more the light bounces, 
the shorter the overall length required to efficiently reflect the 
collected light to the photodetector. The orientation and configuration of 
the mirrors of the collector thus function like a series of thin lenses, 
and the actual physical result is a relatively long focal length and a 
correspondingly large depth of field within a relatively short space. 
The light which is reflected from the surface of the article may include 
both specularly reflected light and diffused or scattered light. These 
reflected light components or light paths are separately collected by 
line-to-spot collectors and converted by respective photodetectors to 
electrical signals for analysis to obtain information about the surface 
characteristics of the article, such as defects or flaws. This particle 
detection system may thus be advantageously used for collecting light that 
is specularly reflected or scattered from the surface of the article. 
The surface inspection system of this invention may also provide an 
underside or edge detector for detecting the light reflected or scattered 
from an edge of the article. The edge detector may then provide additional 
information to the collector, such as in the form of a timing signal, 
about the relative position of the scan line with respect to the article 
being scanned. 
In an alternate embodiment, the specularly reflected light concentrated 
into a spot may be split into two light paths by a beam splitter, the 
first light path defining the specular or far field light and the second 
light path defining the near field light. Separate photodetectors then 
detect the light from the specular field and the near field to provide 
additional information about the surface of the scanned article. 
The invention also provides a method of inspecting the surface of an 
article for particles or flaws. The method of inspecting includes the 
steps of scanning a laser beam along a predetermined scan line across the 
surface of the article, collectively receiving light reflected from the 
article surface along the scan line with a plurality of mirrors so as to 
concentrate the reflected light from a line into a spot.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS 
The present invention will be described more fully hereinafter with 
reference to the accompanying drawings in which illustrated 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 embodiments 
set forth herein; rather, these 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. 
Referring now to the drawings, FIGS. 1 and 2 show a surface inspection 
system, broadly designated at 10, according to the present invention. A 
laser 11 generates a laser beam of light, illustrated by dashed lines B, 
that is reflected and refracted by use of a plurality of mirrors and 
lenses arranged in a series and designated at 12, 13, 14, and 15. The 
mirrors and lenses 12, 13, 14, 15 transfer the light B to a scanning head 
16. The scanning head 16 has a mirror (not shown) which is mounted for 
movement to thereby cause the beam of light B to move in a repeating scan 
pattern and thereby trace a predetermined scan line. The scanning head 16 
is preferably an electromagnetic resonant scanner as shown, but other 
means for scanning the laser beam B apparent to those skilled in the art, 
such as a polygonal rotating mirror or a piezoelectric scanner, may also 
be used. 
The light B from the scanning head 16, in turn, is transmitted to a folded 
optical cell 20. The folded optical cell 20 has a housing 21 to which is 
mounted a flat first mirror 22 positioned for receiving the light B from 
the scanning head 16 and a curved second mirror 23 oriented with respect 
to the first mirror 22 to receive the light reflected from the first 
mirror 22. The first 22 and second mirrors 23 are configured and oriented 
so as to reflectively form a scan line L across the surface of an article, 
such as a silicon wafer designated at W. The reflective surfaces of the 
mirrors 22, 23 are mounted in opposed spaced apart relation to one another 
so that the laser beam B is reflected from each of the reflective surfaces 
a plurality of times prior to finally emerging from the cell, whereupon 
the beam B is directed downwardly onto the surface of the inspection 
target or article W. The article W may be a wafer formed of silicon or 
other semiconductor materials or may be another type of article apparent 
to those skilled in the art. 
The number of bounces by the beam B within the folded optical cell 20 can 
be determined by the entry and exit angles of the scan beam B. By folding 
the beam B within the folded optical cell 20, such that it is reflected 
from each of the reflective surfaces a plurality of times, it is possible 
to significantly increase the overall effective length of travel of the 
laser beam between the scanning head 16 and the article W, i.e., the focal 
length, within a very compact apparatus. The folded optical cell 20 thus 
functions like a series of thin lenses, and the actual physical result is 
a relatively long focal length and a correspondingly large depth of field 
within a relatively short space. 
By using a concave curved mirror 22 as one of the reflective surfaces of 
the optical cell 20 in combination with the planar or flat mirror 23, the 
optical cell 20 also converts the scanning path of the beam B into a 
substantially collimated or parallel scan. Thus, the scanning beam B 
remains substantially perpendicular to the inspection surface as it moves 
across the surface of the article W. Alternatively, the folded optical 
cell 20 may employ a pair of curved mirrors, and the optical cell can be 
set to produce either a parallel, a divergent, or a convergent scan 
pattern. The particular curvature of the curved mirror 22 and the spacing 
with respect to the flat mirror 23 depend upon the specific details of the 
particular scanning system. The particular details and spacing arrangement 
of such a system may be seen in commonly-owned U.S. Pat. No. 4,630,276 
which is hereby incorporated herein by reference. 
As further illustrated in FIGS. 1 and 2, the laser beam B scans along the 
predetermined scan line L across the surface of the article W and strikes 
the article at a predetermined angle of incidence with respect to the 
surface of the article. The beam B is reflected from the surface of the 
article W at an angle equal to the angle of incidence. Any defects, debris 
or irregularities at the surface of the article will cause scattering of 
the incident beam. Thus, the light B which is reflected from the surface 
of the wafer W may include both specularly reflected light and diffused or 
scattered light. These reflected light components or light paths are 
separately collected by the line-to-spot collectors 40, 50 and converted 
by respective photodetectors to electrical signals for analysis by a 
control system 74 to obtain information about the surface characteristics 
of the article W, such as defects or flaws. Suitable means, such as a 
conveyor, is provided for advancing the article W along a predetermined 
path of travel, as indicated by the arrow in FIG. 2, transversely of the 
scan line L of the laser beam B with the surface of the article W located 
in a predetermined target plane. The article W is advanced under the laser 
beam B so that the entire surface of the article W may be scanned. 
A line-to-spot collector 40 positioned above the surface of the article W 
collects the specularly reflected light reflected from the surface of the 
article W and another line-to-spot collector 50 positioned above the 
surface of the article W collects the scattered light scattered from the 
surface of the article W as the article W moves along the predetermined 
path of travel. A lens 70 positioned above the surface of the article W 
refracts the light scattered from the surface to thereby more effectively 
collect the scattered light over a relatively large collection angle as it 
is directed to the collector 50. The collectors 40, 50 have photodetectors 
45, 55, 56, shown in the form of a photomultiplier tube ("PMT") or a 
charge coupled device ("CCD"), positioned for receiving the collected 
light. Also, when the laser beam B scans light across the edges of the 
article W, an edge collector or edge detector 30, located on the opposite 
side of the article W from the laser scan line, receives the laser beam as 
it passes beyond the edges of the article W and thus detects the edges of 
the article W. The edge detector 30 generally provides a timing signal to 
the control system 74 in order to reference the edges of the article W as 
it passes along the scan line L. The edge detector 30 may have a similar 
construction as the line-to-spot collectors 40, 50 and is described in 
further detail later herein. 
FIG. 3 shows the construction and operation of one of the line-to-spot 
collectors 40 in greater detail. The specularly reflected light path 
generally defines a line source of light. The collector 40 receives the 
line of light reflected from the surface of the article W along the scan 
line L and concentrates it into a spot at the entrance to photodetector 
45. The collector 40 has a housing 41, to which is mounted a relatively 
curved first mirror 42 positioned for receiving light reflected from the 
article surface and a relatively flat second mirror 43 and oriented with 
respect to the first mirror 42 to receive light reflected from the first 
mirror 42. A pair of adjustment rods 49 connect to the first mirror 42 and 
the upper portion of the housing 41 and are matingly received by a 
corresponding pair of adjustment knobs 112, also connected to the upper 
portion of the housing 41. By use of the adjustment knobs 112 and the 
adjustment rods 49, the first mirror 42 may be positioned for more 
effective collection of light reflected from the surface of the article W. 
The reflective surfaces of the mirrors 42, 43 of the collector 40 are 
mounted in opposed spaced apart relation to one another so that the 
specularly reflected line of light is reflected from each of the 
reflective surfaces a plurality of times prior to finally emerging from 
the cell, whereupon the beam B is directed upwardly onto the surface of 
the photodetector 45. The number of bounces by the beam B within the 
collector 40 can be determined by the entry and exit angles of the 
specularly reflected light path. The mirrors 42, 43 thus function as a 
folded optical cell, similar to the folded optical cell 20 described above 
except in a reverse orientation. By folding the light path within the 
collector 40, it is possible to significantly increase the overall 
effective length of travel of the specularly reflected light between the 
article W and the photodetector 45, i.e., the focal length, within a very 
compact apparatus. The more the light bounces between the mirrors 42, 43 
the shorter the overall length required to efficiently reflect the 
collected light to the photodetector 45. The folded optical cell of the 
collector 40 thus also functions like a series of thin lenses, and the 
actual physical result is a relatively long focal length and a 
correspondingly large depth of field within a relatively short space. The 
curvature of the first mirror 42 in cooperation with the flat second 
mirror 43 causes the collected light from the scan line L to be focused 
into a predetermined spot at the photodetector 45. The configuration and 
orientation of the mirrors 42, 43 are such that the light reflected into 
the line-to-spot collector 40 is compactly and efficiently collected so 
that the amount of light lost in the transfer process to the photodetector 
is minimized. The other line-to-spot collectors 30, 50 shown in FIGS. 1 
and 2 are constructed in a manner similar to the collector 40 just 
described and, therefore, for brevity will not be further described in 
detail. 
Again referring to FIG. 3, the photodetector 45, such as a photomultiplier 
tube ("PMT") or charge coupled device ("CCD"), for the collector 40 is 
positioned for receiving the thus formed spot of light. The specularly 
reflected light component or light path is separately collected and 
converted by the photodetector 45 to an electrical signal for analysis to 
obtain information about the surface characteristics of the article W. An 
optical element or beam splitter 44, such as a silvered mirror, and a 
light trap 91 may be provided in the light path. The beam splitter 44 and 
the light trap 91 are oriented in such a manner to prevent light from 
scattering backwards into the collected light path thereby reducing the 
background noise received by the photodetector 45. The light trap 91 has a 
darkened interior surface for absorbing the portion of light split into 
the light trap 91. An alignment light 92 also cooperates with the light 
trap 91 and the beam splitter 44 for use in transmitting light in the 
opposite direction to facilitate alignment of the mirrors 42, 43 so as to 
collect the specularly reflected light. The photodetector 45 mounts to an 
electronic circuit board 100 for processing the electrical information 
transmitted from the photodetector 45 about the surface of the article W. 
The collector 50 is positioned above the surface of the article W for 
collecting the scattered light reflected from the surface of the article 
W. The scattered light which is diffused from the inspection surface along 
the scan line L defines a line source of light. The collector 50 is 
similarly constructed like the collector 40 of FIG. 3 so that the line 
source of light is concentrated into a spot. The line-to-spot collector 50 
for the scattered light as shown in FIGS. 1 and 2, however, has a beam 
splitter 54 for splitting the light into first and second light paths. The 
light in the first and second light paths is respectively detected by a 
first photodetector 55, shown as a CCD, and a second photodetector 56, 
shown as a PMT. The electrical signals produced from the first and second 
photodetectors 55, 56 are then combined so as to provide better signal 
recognition for indicating flaws or defects in the article W. The 
information collected by the respective photodetectors 45, 54, 55 of the 
line-to-spot collectors 40, 50 may be processed via suitable interface 
electronics and computer means 74 (FIG. 1) to provide important 
information about the nature, severity and location of the defects or 
flaws present on the surface of the article W. 
Referring again to FIGS. 1 and 2, the edge detector 30 is positioned on the 
underside of the article W for detecting the edges of the article W as it 
passes through the laser scan line L. Light collected from the edge of the 
article W is generally collected from a line and is concentrated into a 
spot for detection. 
Like the collectors 40, 50 in the specularly reflected and scattered light 
paths, the edge detector 30 has a housing 31 with a curved first mirror 32 
mounted thereto and positioned for receiving the light from an edge of the 
article W and a flat second mirror 33 mounted to the housing 31 and so 
oriented with respect to the first mirror 32 to receive the light from the 
first mirror 32. The first 32 and second 33 mirrors of the edge detector 
30 are also configured and oriented so as to concentrate the reflected 
light from a line into a spot. A photodetector 35, shown as a CCD, is 
positioned for receiving the thus formed spot of light. Like the 
collectors 40, 50, the edge detector 30 has the first mirror 32 preferably 
curved and the second mirror 33 preferably flat such that the beam B of 
light has a plurality of bounces between the reflective mirrored surfaces. 
The collected light from the CCD 35 may then be translated into 
information, such as a timing signal, about the location of the edges of 
the wafer W. This information may then be communicated to the reflected 
and scattered light detection circuits of control system 74 for 
recognition of edges of the article W relative to the movement the article 
W along the scan line L. It will be apparent to those skilled in the art 
that other types of edge detectors may be used in combination with one or 
more of the collectors 40, 50 for the surface inspection system 10 as 
described above. 
A second embodiment of the collector 40' according to the present invention 
is shown in FIGS. 4 and 5 with like elements of FIGS. 2 and 3 having a 
prime (') designation. The collector 40' also concentrates the specularly 
reflected light from a line into a spot similar to the collector 40 
described above with reference to FIG. 3. After the collimated light 
reflects from the mirrors 42', 43', however, the light is split into two 
photodetectors 45', 46 by a beam splitter 44' wherein one detector 45' 
detects the specular field of light and the other detector 46 detects the 
near field of light. 
The collector 40' of the second embodiment, as shown in the perspective 
view of FIG. 5, has a prism 44' for splitting the specularly reflected 
light received from the second mirror 43'. The split light for the 
specular field passes through a spatial filter 48 connected to the 
electronic circuit board 100' by rods 110 and is received by a 
photodetector 45'. The spatial filter 48 for the specular field is 
generally rectangular in shape and has a circular opening in a medial 
portion to provide passage of light through only the medial portion and 
therefrom to be detected. Light split to the near field passes through a 
spatial filter 47 connected to the housing 41' by rods 111 and is received 
by a photodetector 46. The spatial filter 47 for the near field is also 
generally rectangular in shape, but in contrast to the specular field 
spatial filter 48, the near field spatial filter 47 has a circular stop in 
a medial portion and an opening extending around the circular stop to 
provide passage of light only through the opening around the circular 
stop. The electrical signals produced from the two photodetectors 45' and 
46 may then be combined to improve the signal recognition of defects, 
particles, or flaws on the surface of the article W, such as the 
signal-to-noise ratio of the signals. 
From the detailed description, and with reference to the drawings, a method 
of inspecting the surface of a article for particles or flaws is also 
provided wherein the laser beam B scans along a predetermined scan line L 
across the surface of the article W. The edges of the article W are 
detected with the edge detector 30 as the laser beam B scans across the 
surface thereof. The light reflected from the article surface along the 
scan line L is collected with the collectors 40, 50 having a plurality of 
mirrors, such as 42, 43 or 52, 53, so as to concentrate the reflected 
light from a line into a spot. The thus formed spot of light may then be 
detected by the photodetectors 45, 54, 55. 
In an alternative method, the thus formed spot of light may be split and 
concentrated into two light paths by a beam splitter 44', such as a 
silvered mirror, the first light path defining a specular field and the 
second light path defining a near field. The specular field light and the 
near field light may then be detected with respective photodetectors, such 
as 45', 46 shown in FIGS. 4 and 5. The output signal of the specular field 
photodetector 45' and the output signal of the near field photodetector 46 
may be combined to thereby improve the recognition of flaws or defects on 
the surface of the article W, such as by increasing the signal-to-noise 
ratio of the electrical signals produced therefrom. 
In the drawings and specification, there have been disclosed preferred 
embodiments of the invention and, although specific terms are employed, 
they are used in a generic and descriptive sense only and not for the 
purposes of limitation, the scope of the invention being set forth in the 
following claims.