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Claims  |
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What is claimed is:
1. An apparatus for inspecting a surface, comprising:
a light source providing a light beam in an incident direction to a spot on
said surface, said spot tracing a line on said surface as time progresses
to illuminate spots along the line; and
a collection system for collecting in directions at a substantially
constant azimuth angle to the incident direction light scattered by the
illuminated spots, said system rejecting scattered light not substantially
at said constant azimuth angle from such spots, said azimuth angle being
different from 90 degrees, and said directions for collecting light being
transverse to a plane of the incident direction and a specular reflection
thereof.
2. The apparatus of claim 1, wherein the collection system includes a
collection aperture that spans a spatial extent of said line to collect
light scattered by spots along said line.
3. The apparatus of claim 2, wherein the line covers an entire length or
width of the surface, and wherein the collection aperture collects light
scattered by all spots along said line.
4. The apparatus of claim 1, said collection system comprising a plurality
of optical fibers with axes oriented such that scattered light from the
spots travelling substantially at said azimuth angle to the incident
direction is refracted substantially along the axes of the fibers.
5. The apparatus of claim 4, said collection system further comprising a
filter for filtering light scattered from said spots towards the fibers or
emerging from the fibers.
6. The apparatus of claim 5, said filter having a smaller collection
aperture than collection angles of the fibers.
7. The apparatus of claim 5, said filter comprising a di-electric filter.
8. The apparatus of claim 7, said source supplying light of a predetermined
wavelength, said filter passing light of said wavelength substantially
only at said azimuth angle from the incident direction.
9. The apparatus of claim 5, said filter comprising a baffle or grating.
10. The apparatus of claim 4, said optical fibers arranged in a bundle with
an end face receiving light scattered from the surface, said end face
being substantially parallel to the line.
11. The apparatus of claim 1, said collection system comprising a filter.
12. The apparatus of claim 11, said filter comprising a di-electric filter.
13. The apparatus of claim 12, said source supplying light of a
predetermined wavelength, said filter passing light of said wavelength
substantially only at said azimuth angle to the incident direction.
14. The apparatus of claim 12, said filter having a surface that is
substantially parallel to the line.
15. The apparatus of claim 12, said filter comprising a baffle or grating.
16. The apparatus of claim 1, said surface being that of a semiconductor
wafer having patterned streets thereon, said incident direction being
substantially parallel to the streets.
17. The apparatus of claim 1, further comprising means for limiting passage
of light in the elevation direction.
18. The apparatus of claim 17, said limiting means comprising a prism or an
aperture interposed between the line and the collection system.
19. A method for collection light from a surface, comprising the steps of:
providing a light beam in an incident direction to illuminate a spot on
said surface, said spot tracing a line on said surface as time progresses
to illuminate spots along the line; and
collecting in directions at a substantially constant azimuth angle to the
incident direction light scattered by the illuminated spots, and rejecting
scattered light not substantially at said constant azimuth angle from such
spots, said azimuth angle being different from 90 degrees, and said
directions for collecting light being transverse to a plane of the
incident direction and a specular reflection thereof.
20. The method of claim 19, wherein the line covers an entire length or
width of the surface, and wherein the collecting step collects light along
directions that are spatially dispersed across said line to collect light
scattered by spots along said line.
21. The method of claim 19, said surface being that of a semiconductor
wafer having patterned streets thereon, said incident direction being
substantially parallel to the streets.
22. The method of claim 19, said collecting step including passing said
scattered light successively through two elements, wherein each of said
elements has a solid angle of collection about an axis that is
substantially at said azimuth angle to the incident direction.
23. The method of claim 22, said passing step passing said scattered light
through a filter and a plurality of fiberoptic channels.
24. An apparatus for selectively collecting light from a surface traveling
along different directions, comprising:
a source supplying substantially monochromatic light of a predetermined
wavelength to illuminate one or more spots on the surface;
a filter having a surface, said filter including a di-electric material
that passes light from said source substantially only when said light is
at substantially a predetermined angle to the surface of the filter; and
means for changing the orientation of the filter, thereby causing a first
beam originally not passed by the filter to be passed by the filter, and a
second beam originally passed by the filter to be not passed by the
filter, said first and second beams originating from the spots.
25. The apparatus of claim 24, said predetermined angle being different
than 90 degrees.
26. The apparatus of claim 25, wherein said means for changing rotates the
filter.
27. The apparatus of claim 26, wherein said means for changing rotates the
filter to a first position to pass specularly reflected light from the
surface and to a second position to pass non-specularly scattered light.
28. The apparatus of claim 24, said source causing a light beam to trace a
scan line on the surface, said changing means rotating the filter about
said scan line or a line substantially parallel to the scan line.
29. A method for selectively collecting light traveling along different
directions, comprising the steps of:
supplying substantially monochromatic light of a predetermined wavelength
to illuminate one or more spots on the surface;
providing a filter having a surface, said filter including a di-electric
material that passes light from said source substantially only when said
light is at substantially a predetermined angle to the surface of the
filter; and
changing the orientation of the filter, thereby causing a first beam
originally not passed by the filter to be passed by the filter, and a
second beam originally passed by the filter to be not passed by the
filter, said first and second beams originating from the one or more spots
on the surface.
30. The method of claim 29, said angle being different than 90 degrees.
31. The method of claim 30, wherein said changing step rotates the filter.
32. The method of claim 31, wherein said changing step rotates the filter
to a first position to pass specularly reflected light from the surface
and to a second position to pass non-specularly scattered light.
33. The method of claim 29, said supplying step causing a light beam to
trace a scan line on the surface, said changing step rotating the filter
about said scan line or a line substantially parallel to the scan line.
34. An apparatus for detecting anomalies of the surface, comprising:
a light source providing a light beam in an incident direction to a spot on
said surface, said spot tracing a line on said surface as time progresses
to illuminate spots along the line; and
a collection system for collecting at a substantially constant azimuth
angle to the incident direction light scattered by the illuminated spots,
said system rejecting scattered light not substantially at said constant
azimuth angle from such spots, said azimuth angle being different from 0
and 90 degrees; and
a detector receiving light from the aperture from all of said directions. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
This invention relates in general to systems for particle detection on
surfaces and, in particular, to a particle detection system employing a
subsystem for collecting light from surfaces within predetermined
apertures or collection angles.
In the process of manufacturing devices such as semiconductor wafers, flat
panel displays, photomasks, ceramic substrates and other devices, it is
important to detect contaminant particles on the surface of an object
using the principle of light scattering. In U.S. Pat. No. 4,898,471
assigned to Tencor Instruments, the assignee of the present application, a
system for detecting particles and other defects on a patterned
semiconductor wafer, photomask or the like is disclosed. A polarizing
filter is used in the system to polarize the beam of light in the
direction substantially parallel to the surface of the patterned
semiconductor wafer to be examined. The beam is enlarged in
cross-sectional diameter by a beam expander placed along the path of the
beam after the polarizing filter. The beam is then caused to scan by a
deflection mirror. The scanning beam is then focused on the patterned
wafer at a grazing angle of incidence along an incident direction that is
substantially parallel to the patterned streets formed on the wafer. A
light collection system for detecting scattered light is positioned in an
azimuth direction in a range from about 80.degree.-100.degree. from the
incident direction of the scanning beam. The light collection system
includes a lens for focusing the scattered light, a polarizing filter
oriented in a direction substantially parallel to the surface of the
patterned wafer and a photomultiplier tube.
In many semiconductor wafers, such as those formed in manufacturing logic
devices, 45.degree. geometries are prevalent, either as interconnects or
as repeated feature edges. If the 45.degree. geometries are small in size
relative to the wavelength of the incident scanning beam and spot size,
the diffraction generated by such geometries can be filtered in the
Fourier plane with a programmed blockage. If such geometries are
irregularly shaped or are widely spaced apart, it may be difficult to
filter the diffraction pattern at the Fourier plane.
Given a known diffraction pattern from the geometries of the pattern on the
semiconductor wafer, it may be possible to design a light collection
system that would avoid the high intensity pattern scatter in specific
directions. However, since the path of the scanning beam covers the entire
length or width of the wafer, the scattered light can originate from any
point of the long scan line. If the scanning beam has a cross-section that
is large relative to the size of the particles being detected, specular
reflection and some components of reflection from patterned features would
contain many times the light radiated from the particles. Thus the light
collection system would have to be designed to avoid the specular
reflection and reflection from patterned features originating from any
point on the long scan line.
To detect defects on the patterned wafer, one existing technique is to
construct templates from the scattered light from individual die of the
wafer and comparing the template to the scattered light from other die on
the wafer. Both pattern features and contamination have specific and often
extremely selective radiation patterns. In order for the comparison to the
template to be meaningful, it is very desirable for the light collection
system to collect light at a constant collection angle; existing particle
detection systems for patterned wafers have used Fourier plane stops to
limit the aperture of the system. While this can be achieved for any
collection angle, due to the long scan length discussed above, and the
need to avoid specular beams from geometries such as the 45.degree.
geometries on the wafers, it may be difficult to design a light collection
system that is practical. In many instances, the optical system simply
becomes too large for users.
From the above, none of the particle detection systems and the light
collection subsystems they employ are entirely satisfactory. It is
therefore desirable to provide an improved particle detection system and
an improved light collection subsystem in which the above-described
difficulties are alleviated.
SUMMARY OF THE INVENTION
One aspect of the invention is based on the observation that, by using one
or more optical fibers to collect the scattered light and orienting the
optical fibers at predetermined directions relative to the incident
direction of the scanning beam to receive the scattered light at an
azimuth angle different from 90.degree., many of the difficulties
described above can be avoided. The one or more optical fibers inherently
have capture or collection angles in which they collect light, so that
light originating outside of such capture or collection angles will be
severely attenuated and fail to emerge in significant intensity when
passed through the fibers. Therefore, by orienting the optical fibers at
predetermined directions relative to the incident direction of the
scanning beam, it is possible to reduce the amount of high intensity
reflections and scatter reaching the fibers while collecting signal from
the desired features. In particular, to avoid the reflections caused by
45.degree. geometries on the patterned wafer, the optical fibers are
oriented at such directions that they will receive light scattered at
azimuth angles different from 90.degree..
Thus one aspect of the invention is directed towards an apparatus for
detecting particles on a surface, comprising a source supplying light
along an incident direction to a region on the surface and one or more
optical fibers oriented in predetermined directions relative to the
incident direction to receive light scattered from said region at an
azimuth angle different from 90.degree.. A related aspect of the invention
is directed towards a method for detecting particles on a patterned
surface of a semiconductor material. The method comprises supplying light
along an incident direction to a region on the surface, and orienting one
or more optical fibers at predetermined angles to the incident direction
to receive light scattered from said region at an azimuth angle different
from 90.degree..
A bandpass filter composed of a di-electric stack can be designed to
transmit light of a specific wavelength at a specific range of angles
relative to the direction normal to the surface of the di-electric stack,
while reflecting light of the same wavelength that impinges the stack
surface at angles outside the range. This range of passed light creates an
angular collection aperture, and is referred to in this application as the
aperture or collection aperture. This filter may be used to control the
aperture of a light collection system in order to reduce the amount of
undesirable light collected by the light collection system. Thus another
aspect of the invention is directed towards an apparatus for collecting
light. The apparatus comprises a filter including a di-electric material
that passes light of a predetermined wavelength substantially only within
said aperture, and an optical system collecting light that passed through
the filter.
Depending on which portion of the end face of an optical fiber that a
specular beam strikes, the specular beam may still emerge from the fiber
at the other end even if the specular beam is outside the capture or
collection angle of the fiber. Thus detection accuracy of the system can
be improved by employing means for controlling the aperture or collection
aperture of the scattered light either before or after the light passes
through the fiber. Thus another aspect of the invention is directed
towards an apparatus for detecting particles on the surface, comprising a
source supplying light to a region on the surface, and one or more optical
fibers oriented to receive light scattered from said region. The apparatus
further comprises means for controlling the aperture of the scattered
light before or after such light passes through the one or more fibers.
Another aspect of the invention is directed towards the light collection
subsystem of the particle detection apparatus described immediately above.
Yet another aspect of the invention is therefore directed towards an
apparatus for collecting light comprising one or more optical fibers
oriented to receive light within a capture or collection angle and means
for controlling the aperture of the scattered light before or after such
light passes through the one or more fibers.
An aspect of the invention related to that above is directed towards a
method for detecting particles on the surface of a semiconductor material,
comprising supplying light to a region on the surface, orienting one or
more optical fibers to receive light scattered from said region and
controlling the aperture of the scattered light before or after such light
passes through the one or more fibers. Yet another related aspect of the
invention is directed towards a method for collecting light, comprising
orienting one or more optical fibers to receive light within an aperture
or collection angle, and controlling the aperture of the scattered light
before or after such light passes through the one or more fibers.
With the elevation angle of light collection constrained by the optical
fiber geometry or other apertures, it is desirable to be able to alter the
azimuth angle of light collection by tilting the di-electric filter to
change the incident angle of the light beam relative to the light
collecting surface of the filter. If the tilting causes the light
collecting surface of the filter to have a different orientation in the
elevation plane, the light beam will impinge the light collecting surface
at a different incident angle, thereby altering the collection aperture of
the system.
Thus one more aspect of the invention is directed towards an apparatus for
collecting light, comprising a filter including a di-electric material
that passes light of a predetermined wavelength substantially only when
said light is within a predetermined collection aperture, and means for
moving the filter to alter the orientation of the material, thereby
altering the collection aperture. Another related aspect of the invention
is directed towards a method for collecting light, comprising providing a
filter including a di-electric material that passes light of a
predetermined wavelength substantially only when said light is within a
predetermined collection aperture, and moving the material to alter its
orientation, thereby altering the collection aperture.
The above-described feature where the di-electric material is moved to
alter the collection aperture may be used in a system for detecting
particles. Therefore, another aspect of the invention is directed towards
an apparatus for detecting particles on the surface, comprising a source
supplying a scanning beam of light to the surface, causing scattered light
to travel along different directions, a filter including a di-electric
material that passes light of a predetermined wavelength substantially
only when said light is within a collection aperture, and means for moving
the material to alter its orientation, thereby altering the collection
aperture. Yet another aspect of the invention is directed towards a method
for detecting particles on the surface, comprising supplying a scanning
beam of light to the surface, causing scattered light that travels along
different directions, providing a filter including a di-electric material
that passes the scattered light of a predetermined wavelength
substantially only when said scattered light is within a predetermined
collection aperture, and moving the material to alter its orientation,
thereby altering the collection aperture.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view of an end portion of an optical fiber
illustrating its collection aperture or capture angle to illustrate the
invention.
FIG. 2 is a perspective view of a particle detection system to illustrate
the preferred embodiment of the invention.
FIG. 3 is a top view of a portion of the system of FIG. 2.
FIG. 4 is a schematic view of a collection of optical fibers in a bundle
and the paths of scattered light towards the bundle to illustrate the
invention.
FIG. 5 is a schematic view of a filter made of a di-electric material and
the paths of light beams to illustrate another aspect of the invention.
FIG. 6 is a schematic view of a section of an optical fiber and the paths
of light emerging from the fiber and a filter for filtering the light
emerging from the fiber to illustrate another aspect of the invention.
FIG. 7 is a schematic view of the filter of FIG. 5 and a light collection
system to illustrate the invention.
FIGS. 8A-8C are schematic views of control systems for controlling the
elevation collection angle to illustrate the invention.
FIGS. 9A, 9B are schematic views illustrating how the di-electric filter of
FIG. 5 can be rotated to control the collection angle or aperture for
illustrating another aspect of the invention.
FIG. 10 is a side view showing in more detail the control mechanism of FIG.
8C to illustrate the aspect of FIGS. 9A, 9B.
FIGS. 11 and 12 are schematic views of respectively a baffle and a grading
type aperture control device that can be used instead of a di-electric
filter to illustrate yet other aspects of the invention.
FIG. 13 is an isometric view parallel to that of FIG. 6 illustrating an
aperture control device that can be used to replace the di-electric filter
of FIG. 6.
FIGS. 14A, 14B, 14C are cross-sectional views of a section of an optical
fiber, each with a different cladding composition or material to
illustrate another aspect of the invention.
For simplicity in description, identical components are labeled by the same
numerals in this application.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1 is a schematic view of an end portion of an optical fiber to
illustrate its capture or collection angle. Fiber 20 has a capture or
collection angle theta. As shown in FIG. 1, the capture angle theta is
bounded by the two extreme positions 22, 24 of light rays that would
impinge upon a center portion of the end face 20a of fiber 20, pass
through the fiber to emerge at the other end of the fiber with significant
intensity. Balloon 26 is a graphical plot representing the amount of light
transmitted at a corresponding angle to the axis A of the fiber. Thus the
light transmission at points directly on the axis A would be 100%, with
transmission loss gradually increasing with the angle away from axis A
until the extreme positions 22, 24 are reached, beyond which
insignificantly amount of light will be transmitted by the fiber. FIG. 1
illustrates the passage of light rays that impinge upon end face 20a
outside of the collection or capture angle. As illustrated in FIG. 1, each
internal reflection within fiber 20 of light rays 28 slightly beyond the
extreme positions 22, 24 would cause a loss of light intensity. For this
reason, light rays that impinge end face 20a at an oblique angle are
likely to be totally lost and unlikely to pass through fiber 20 to emerge
at the other end.
In the description below, the features of the invention are illustrated by
reference to patterned surfaces, such as surfaces of patterned wafers,
although the invention is just as applicable to bare wafer detection and
light collection. FIG. 2 is an isometric view of a particle detection
system 40 including a light collection subsystem 70 to illustrate the
preferred embodiment of the invention. As shown in FIG. 2, a laser beam 50
is generated by a laser scan source 52 which can cause beam 50 to scan a
surface. Laser beam 50 is directed towards a spot or portion 54 of a wafer
surface 56. As known to those skilled in the art and discussed in U.S.
Pat. No. 4,898,471 referenced above, laser beam 50 is caused to move in a
scanning motion so that spot 54 traces a scan line 60 on wafer surface 56,
along the X direction, parallel to streets between die.
A stage (not shown) moves the wafer along the Y direction so that spot 54
would in time be directed to cover every portion of the surface 56 of the
wafer. The specular reflection of beam 50 from the unpatterned portions of
surface 56 is shown along dotted lines 62. Aside from the specular
reflection from the unpatterned portions of surface 56, the patterned
portions as well as contaminant particles that are illuminated within spot
54 scatter light. A portion of such scattered light is collected by a
light collection subsystem 70, and the light so collected is directed
towards a detector 72 for analysis of wafer surface 56. Subsystem 70
includes a filter 74 and bundle of optical fibers 76.
Certain particular geometries of devices on surface 56 would cause high
intensity specular beams or defraction patterns to be emitted at
particular directions relative to the incident direction of beam 50. For
example, 45.degree. geometries in spot 54 would cause strong pattern light
signals to be directed in a cone having strong intensity components in
directions at 90 degrees to the illuminating beam 50, such as directions
at 90 degrees azimuth angle. One of the aspects of the invention is based
on the observation that optical fibers have collection or capture angles
and therefore can be placed at the desired locations to avoid the strong
pattern signals and to maximize the chance of detecting anomalies on the
wafer surface 56.
FIG. 3 is a top view of a portion of the light collection system 70, the
wafer and a schematic representation of beam 50 and reflection 62 of FIG.
2, where a portion of the bundle 76 has been cut away to expose the
reflection 62. Beam 50 of course has a certain width and so does its
specular reflection 62. Line 80 within beam 50 may be chosen to represent
the incident direction (e.g., as an average direction of the beam) of the
beam 50. Similarly, the scattered light 82 that is collected by system 70
also has a spread, but its average direction can again be represented by a
line 84. The angle phi between the incident direction 80 and direction 84
as seen from the top view is known as the azimuth angle of the scattered
light beam 82, and the term "azimuth angle" will be used with such meaning
in this application. Beam 82 also has an elevation angle that it makes
with surface 56 when viewed from the side. If phi is less than 90 degrees,
beam 82 is a forward scattering beam; if phi is greater than 90 degrees,
beam 82 is a back scattering beam. The collection scheme of this invention
is symmetrical and can be used for both forward as well as back scattering
systems.
FIG. 4 is a schematic view of the incident beam 50, scan line 60, the
fiberoptic bundle 76 of system 70, but omitting filter 74, showing the
passage of scattered light through the optical fibers to illustrate the
invention. In the preferred embodiment shown in FIG. 2, the light
collection system 70 includes a filter 74 and a fiberoptic bundle 76. In
some embodiments, it may be possible to omit the filter so that the
scattered light would impinge directly onto the end faces 76a of the
fibers in bundle 76, as shown in FIG. 4. For simplicity, the incident beam
50 and the scattered beams are represented in FIG. 4 as single lines, it
being understood that these beams of course have certain widths. In
general the collection of specular beams is avoided by using a different
elevation than where most specular reflection occurs; scattering of course
occurs in any direction dependent on the surface. As shown in FIG. 4,
light impinging on spot 54 can be scattered in a number of directions
towards the bundle 76. Bundle 76 includes a number of optical fibers with
axes A.
Preferably and as shown in FIG. 4, the end faces 76a of the fibers are not
perpendicular to their axes A to allow better collection of scattered
light along desired azimuth angles. Light actually collected is determined
by intersecting the fiber end face and then by angularly dependent
transmission by the fiber. In FIG. 4, the fibers in bundle 76 are
constructed so that the passage of light at an azimuth of 45 degrees
through the bundle would be optimal. The azimuth angle of beam 82 is about
45.degree.. The end faces 76a of the fibers 76 are at such angles with
axes A, and the fibers have such index of refraction that when beam 82
impinges upon end face 102 of fiber 100, such beam would be refracted into
beam 82' that travels along or in a direction parallel to the axis A of
fiber 100. End face 102 is the boundary between the fiber 100 and air,
which has index of refraction of 1. Thus according to Snell's Law, where
the index of refraction of the fiberoptic material is 1.6, end face 102 is
approximately at 65.degree. angle to axis A. The reason for finishing the
fibers at such angle and as shown is to allow mass polishing and to avoid
introduction of scatter between fibers.
As shown in FIG. 4, light from spot 54 is scattered in many directions
other than direction 84. Such beams also impinge on the end faces 76a of
fibers in bundle 76. Where such scattered light travels in directions far
away from direction 84, these beams are typically outside the capture or
collection angles of the fibers whose end faces they impinge upon, so that
these beams would be severely attenuated through internal reflections and
absorption and would therefore fail to emerge in significant intensity
from the other end 76b of the bundle.
The aperture control effect inherent in the collection or capture angles of
the fibers in bundle 76 may be adequate to discriminate against scattered
light away from the desired directions. In FIG. 4, for example, the
aperture control effect due to collection angles of the fibers is that
only light scattered from spot 54 forming a cone with an axis along
direction 84 will be passed by the bundle, while scattered light along
other directions will not. When beam 50 is scanned along the scan line 60,
spot 54 may move to a new spot 54'. At such location, essentially the same
process will be repeated as described above where the scattered light from
spot 54' within a cone with axis 84' along a 45.degree. azimuth angle will
pass through bundle 76, while scattered light in other directions outside
the cone will be rejected and fail to emerge at end 76b of the bundle.
However, as described below, if the incoming light impinges the fiber ends
at points near the edges of the fibers, such light will pass through the
fibers even though they are outside the collection or capture angles of
the fibers. In the preferred embodiment, it is desirable to employ an
aperture control device for further aperture control, such as filter 74 in
FIG. 2, to only permit light that are on or close to the desired direction
84, 84' to pass and impinge upon end 76 of bundle 76. A filter having such
properties is illustrated in FIG. 5 described below.
A di-electric filter 74 may comprise a stack of different layers of
di-electric material, where the material will reflect most of the light of
a particular wavelength that impinges upon the layer in the normal
direction to the layer but would transmit about 80% of the light of the
same wavelength if the light impinges upon the layer at a different angle
than normal, such as within a range of angles around 45.degree. (defining
the aperture or collection aperture of the filter) as illustrated in FIG.
5. In one embodiment, the filter layer 74 is constructed so that it
reflects most of the light at wavelength of 488 nanometers (nm) that
impinges the layer in a direction normal to the layer but will pass 80% or
more of light of such wavelength that impinges the layer at around
45.degree.. Where the light beam impinges the layer at an angle other than
90.degree., the wavelength as seen by the layer is actually shorter than
the actual wavelength of the light. As illustrated in FIG. 5, beam 110
impinging layer 74 at 90.degree. has wavelength W while beam 112 with the
same wavelength impinging layer 74 at 45.degree. will have an effective
wavelength of 0.707 W instead. Thus 80% of beam 112 will be transmitted by
layer 74 as beam 112' and 20% of the incident beam 112 will be reflected
as a reflected beam 112". Thus if the laser scan source 52 directs a beam
50 at wavelength 488 nm at spot 54, the reflected light from spot 54 would
also be substantially at such wavelength. If such scattered light impinges
upon filter 74 in a normal direction, such as beam 110 in FIG. 5, such
beam would be reflected, whereas 80% of the light scattered from spot 54
that impinges layer 74 at 45.degree. would be transmitted to bundle 76. An
80% transmission rate in the light collection system 70 has been found to
be satisfactory for the purpose of particle detection in system 40 of FIG.
2. It is found that using a filter such as filter 74, only about 10.sup.-7
of the light is transmitted in the normal direction.
The stack of di-electric layers 74 may have a wide range of acceptab | | |
