U.S. patent number RE33,213 [Application Number 07/262,408] was granted by the patent office on 1990-05-08 for light scattering particle detector for wafer processing equipment.
This patent grant is currently assigned to High Yield Technology. Invention is credited to Peter Borden.
United States Patent |
RE33,213 |
Borden |
May 8, 1990 |
Light scattering particle detector for wafer processing
equipment
Abstract
A particle detector includes a laser, a beam shaping lens, and a
pair of mirrors which reflect the shaped laser beam back and forth
between the mirrors a selected number of times in order to create a
sheet of light or light net between the mirrors. The path of the
beam is terminated by a beam stop which contains a photodiode to
monitor beam intensity and thereby system alignment. Light
scattered by a particle falling through the sheet of light is
gathered and transmitted to a photodiode. A peak detector provides
a measure of the peak intensity of light scattered by such a
particle to a microprocessor, which counts the number of particles
falling through the light net in a selected time interval. The
microprocessor also uses the peak intensity to estimate the size of
the particle.
Inventors: |
Borden; Peter (Mountain View,
CA) |
Assignee: |
High Yield Technology (Mountain
View, CA)
|
Family
ID: |
27500739 |
Appl.
No.: |
07/262,408 |
Filed: |
October 25, 1988 |
Related U.S. Patent Documents
|
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
807901 |
Dec 11, 1985 |
|
|
|
|
807395 |
Dec 10, 1985 |
|
|
|
Reissue of: |
907776 |
Sep 16, 1986 |
04739177 |
Apr 19, 1988 |
|
|
Current U.S.
Class: |
250/574; 250/575;
356/338 |
Current CPC
Class: |
G01N
15/0205 (20130101); G01N 21/53 (20130101); G01N
2015/0238 (20130101); G01N 2021/516 (20130101) |
Current International
Class: |
G01N
15/02 (20060101); G01N 21/53 (20060101); G01N
21/47 (20060101); G01N 21/51 (20060101); G01N
015/06 (); G01N 015/07 (); G01N 021/49 () |
Field of
Search: |
;250/573,574,575
;356/336,337,338,339,340,341,342,343 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Nelms; David C.
Assistant Examiner: Oen; William L.
Attorney, Agent or Firm: MacPherson; Alan H. Franklin;
Richard Kallman; Nathan N.
Parent Case Text
CROSS-REFERENCE TO COPENDING APPLICATION
This application is .Iadd.a reissue application for U.S. Pat. No.
4,739,177 which issued on .Iaddend.a continuation in part of
.[.copending.]. U.S. patent application Ser. No. 807,901, filed
Dec. 11, 1985, now abandoned, which is a continuation of the parent
application Ser. No. 807,395 filed Dec. 10, 1985 now abandoned.
Claims
I claim:
1. Apparatus for detecting particles in a vacuum, or air or fluid
environment comprising:
means for providing a light beam having a defined access;
lens means for shaping said light beam to provide a collimated beam
having a predetermined width and height;
first mirror means having a first surface for .Iadd.reflecting said
light beam .Iaddend..[.providing multiple reflections of said light
beam, the angle of incidence of each of said reflections of said
light beam being less than 90.degree..].;
second mirror means having a second surface for .Iadd.reflecting
said light beam .Iaddend..[.providing multiple reflections of said
light beam that is reflected from said first surface, the angle of
incidence of each of said reflections of said second surface being
less than 90.degree..].;
said first and second surfaces of said first and second mirror
means respectively being substantially parallel and spaced from
each other so that .Iadd.said light beam is reflected from said
first surface to said second surface a first multiplicity of times
and from said second surface to said first surface a second
multiplicity of times, said light beam having an angle of incidence
on said first surface and on said second surface of less than
ninety degrees (90.degree.) prior to each reflection from said
first surface and said second surface, said first and said second
surfaces and said means for providing a light beam being arranged
so that .Iaddend.the distance between adjacent points of reflection
on each surface is substantially the dimension of the width of said
light beam.Iadd., said first and second mirror means having highly
reflecting surfaces; .Iaddend.
photodiode means .Iadd.including first and second photodiodes
.Iaddend.for detecting light that is scattered by particles that
traverse said beam, said .Iadd.first and second
.Iaddend.photodiodes being spaced at a distance from each other
greater than the distance between said first and second
.[.reflecting.]. surfaces .Iadd.of said first and second mirror
means. .Iaddend.
2. A particle detector as in claim 1, wherein said first mirror
means and said second mirror means comprise planar dielectric
mirrors.
3. A particle detector as in claim 1, wherein said angles of
incidence are less than or about 15.degree..
4. A particle detector as in claim 1 further comprising a beam stop
for terminating said beam after said beam has been reflected a
plurality of times.
5. A particle detector as in claim 4 wherein said beam stop
includes means for sensing the intensity of light incident on said
beam stop.
6. A particle detector as in claim 5 wherein said beam stop further
includes means for emitting a signal when the intensity of light
incident on said beam stop falls below a preselected value.
7. A particle detector as in claim 1 wherein said means for
providing a beam of light comprises a laser.
8. A particle detector as in claim 1 wherein said first
.Iadd.mirror .Iaddend.means .[.for reflecting said beam of light.].
and said second .Iadd.mirror .Iaddend.means .[.for reflecting said
beam of light.]. each comprises a mirror curved to reduce the
divergence of said beam of light.
9. A particle detector as in claim 1 wherein said lens means
includes a lens .[.having a focal length selected to compensate for
beam divergence.]. positioned between a source of said beam of
light and said first .Iadd.mirror .Iaddend.means .[.for
reflecting.]..Iadd., said lens having a focal length selected to
compensate for beam divergence. .Iaddend.
10. A particle detector as in claim 1 further comprising at least
one member extending outward from .Iadd.at least .Iaddend.one of
said .Iadd.first and second mirror means .Iaddend..[.mirrors.]. to
prevent dust from settling on .Iadd.at least one of .Iaddend.said
.Iadd.first and second surfaces, respectively .Iaddend..[.surface
of said one of said mirrors.]..
11. A particle detector as in claim 1 further comprising at least
one opaque member extending outward from .Iadd.at least
.Iaddend.one of said .Iadd.first and second mirror means
.Iaddend..[.mirrors.]. to prevent light scattered from
imperfections in .Iadd.at least one of .Iaddend.said
.Iadd.respective first and second surfaces .Iaddend..[.surface.].
of said .Iadd.at least .Iaddend.one of said .Iadd.first and second
mirror means .Iaddend..[.mirrors.]. from being detected by said
photodiode .Iadd.means. .Iaddend.
12. A particle detector as in claim 1 wherein said lens means
comprises one or more lenses which produce a beam of light having a
height less than its width.
13. A particle detector as in claim 1 further including means for
chopping said light beam.
14. A particle detector as in claim 1 wherein said photodiode means
for detecting scattered light comprises a collecting mirror, said
collecting mirror focusing said scattered light on .Iadd.one of
.Iaddend.said .Iadd.first and second photodiodes
.Iaddend..[.photodiode means.]..
15. A particle detector as in claim 14 wherein said
.Iadd.photodiode .Iaddend.means for detecting further includes
means for .Iadd.providing .Iaddend..[.sensing.]. a .Iadd.signal
.Iaddend.representative of .[.the.]. .Iadd.a .Iaddend.peak
amplitude of a light signal received by said .Iadd.one of said
first and second photodiodes .Iaddend..[.photodiode.]..
16. A particle detector as in claim 15 wherein said
.Iadd.photodiode .Iaddend.means for detecting further includes an
analog-to-digital converter and a microprocessor, said
analog-to-digital converter providing said microprocessor with
.[.the.]. .Iadd.a .Iaddend.digital representation of said
.Iadd.signal .Iaddend.representative of said peak amplitude of said
light signal received by said photodiode .Iadd.means. .Iaddend.
17. A particle detector as in claim 1 wherein said lens means
comprises a gradient index lens.
18. A particle detector as in claim 1, including a pipe section
having a narrow cavity extending substantially transversely to said
beam of light.
19. A particle detector as in claim 18, including window means
.Iadd.for protecting said first and second surfaces while allowing
said light beam to pass through said window means, said window
means being .Iaddend.disposed adjacent to said pipe section and
between said first surface and said second surface.
20. A particle detector as in claim 18, wherein said pipe section
comprises flanged portions for coupling to external pipes so that
fluids or gases can be passed through said narrow cavity.
Description
FIELD OF THE INVENTION
This invention relates to a particle detector and in particular to
a particle detector for monitoring airborne particles or particles
in a vacuum, or in a fluid environment.
BACKGROUND
As wafer size increases and as device geometry becomes smaller,
particulate detection and control becomes ever more important in
semiconductor processing. Monitoring of particulate levels is
important in processes which take place in an environment of air at
atmospheric pressure, for example exposure of photoresist patterns,
and for processes which take place in a vacuum chamber, for example
deposition of metal films. Particulate contamination can be reduced
for processes which take place in an environment of air at
atmospheric pressure by the use of so called clean rooms which
employ air filtration systems. Even with air filtration systems,
however, processing equipment employs moving parts which generate
particles and monitoring of particulate levels is therefore
desirable for early detection of system breakdowns which produce
excessive particulate levels.
One prior art method for detecting airborne particles is shown in
FIG. 1. Sampled air (indicated by arrow 5 in FIG. 1) is drawn
through narrow transparent tube 6 by a vacuum pump (not shown)
attached to end 6a of cylindrical tube 6. Monochromatic light 1
from a laser (not shown) or white light from a lamp (not shown) is
focused by lens 2 to form a focused beam 3 which passes through
transparent tube 6 at a selected point along the tube. Light
scattered from particles in sampled air 5 drawn through tube 6
which passes through beam 3 is detected by detector 7.
Alternatively an opening (not shown) in tube 6 and an air sheath
may be provided so that the focused beam passes through the opening
in the tube. Detector 7 contains a photomultiplier and its
construction is well known in the art. The scattering intensity is
roughly proportional to particle size. Such systems commonly detect
particles having a mean diameter in a range between 0.1 microns and
7.5 microns and in principle even smaller and larger particles can
be detected using the above system.
This prior art particle monitoring device has several
drawbacks:
(1) It essentially samples air from a point, i.e. the point of the
opening of the tube, and does not provide an adequate measure of
particulate contamination over a wider spacial region. Often in a
semiconductor processing environment, moving parts of various
machinery may produce particles that will not be detected by
sampling at a particular point. Thus prior art particle monitoring
devices do not adequately monitor particles from multiple or
distributed sources.
(2) The prior art monitoring system works in air but not in a
vacuum chamber since it requires a flow of air to carry the
particles.
(3) Particles may stick to the sides of tube 6 and then become
airborne again at a later point in time thereby creating a delay
effect.
(4) The physical end 6b of the tube must be placed physically close
to the point being monitored which may interfere with other
portions of the processing system.
SUMMARY OF THE INVENTION
A particle detector is provided which is suitable for detecting
particles which are present in either air or in a vacuum. In one
embodiment the detector includes a laser and beam shaping lenses
which generate a beam whose height is small compared to its width.
The beam is reflected back and forth between two mirrors a selected
number of times in order to create a light "sheet" or "net" between
the two mirrors. The path of the light is terminated by a "beam
stop" which monitors the intensity of the beam thereby providing a
measure of system alignment. Light scattered from a particle
falling through the light net generated between the two mirrors is
detected by one or more photodiodes. Signals generated by the
photodiodes are amplified and processed by a peak detector. The
peaks above a selected threshold value are counted by a
microprocessor, which calculates particle flux density.
In one embodiment the beam is chopped and a lens is employed to
focus the beam in order to compensate for beam divergence.
Projecting members prevent dust from settling on the reflecting
surfaces of the mirrors and also prevent light scattered by
imperfections in the mirror surface from reaching the photodiodes.
In a specific arrangement, the photocells are located so as to
provide direct viewing of the light sheet between the two
reflecting mirrors thereby enabling the making of a very compact
sensor assembly. Since the reflecting mirrors are moved closer
together in the compact sensor assembly, a significant improvement
in response to the sensed light beam is realized. In another
arrangement which is useful for monitoring particles in high
temperature environments, such as experienced with hot or corrosive
gases or liquids, the sensor assembly includes a narrow, elongated
pipe with attached glass windows to provide a small gap between the
reflecting mirrors. The glass windows serve to protect the optical
system from corrosion and heat. The housing for the sensor assembly
is water-cooled to reduce thermal effects.
Two or more particle detectors of this invention may be ganged
together to provide detection of particles falling through a large
area, for example 8 inches by 8 inches.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows an airborne particle detector of the prior art;
FIG. 2a shows a top view of the particle detector of the present
invention;
FIG. 2b shows the shape of the beam emerging from the beam expander
shown in FIG. 2a;
FIG. 2c shows a partial side view of the particle detector shown in
FIG. 2a;
FIG. 2d shows an alternate path for the light employed in the
particle detector;
FIG. 2e shows the path of the light scattered along 15.degree. rays
reflected by parabolic mirror 21 and focused on photodiode PD.sub.1
shown in FIG. 2a and FIG. 2c;
FIG. 3a shows the scattering cross section for a spherical particle
into an angular region between .theta. and .theta.+5.degree.;
FIG. 3b shows the angle .theta. and .theta.+5.degree. employed in
FIG. 3a;
FIG. 4 shows a block diagram of the circuitry used to process the
signal received by photodiode PD.sub.1 ;
FIG. 5a shows a typical output signal of amplifier 34b shown in
FIG. 4;
FIG. 5b shows the positive envelope of the signal shown in FIG.
5a;
FIG. 6 illustrates beam divergence as a function of path
length;
FIG. 7a shows integrated scatter cross section versus angular
region;
FIG. 7b shows the angular region between a cone having angle
10.degree. and a cone having angle .phi.;
FIG. 8 shows a lens arrangement for compensating for beam
divergence;
FIG. 9 shows an alternate arrangement for compensating for beam
divergence;
FIG. 10 is a top view of a compact direct view sensor assembly,
that is an alternative embodiment of this invention;
FIG. 11 is a side view of the sensor assembly of FIG. 10;
FIG. 12 is an end view of the sensor assembly illustrated in FIG.
11;
FIG. 13 is a top view of another alternative embodiment of the
invention employing a compact sensor assembly including a pipe
structure; and
FIG. 14 is a side view of the sensor assembly of FIG. 13.
DETAILED DESCRIPTION
FIG. 2a shows a plan view (not to scale) and FIG. 2c shows a side
view of one embodiment of the particle monitor of the present
invention. Laser 10 is preferably a semiconductor laser, for
example an AlGaAs laser operating at a wavelength of 820 nm. RCA
laser No. C86000E and Hitachi laser No. HL8312E are suitable for
this purpose. Other sources of light (not necessarily
monochromatic) may also be employed with this invention. Beam 11
which emerges from the P-N junction of semiconductor laser 10 is
shaped (collimated) by cylindrical beam shaping lenses 12 and 13
which are coupled to shape the beam in the horizontal and vertical
planes. Beam 14 which emerges from lens 13 is shown in more detail
in FIG. 2b which shows a cross section of laser beam 14 along line
A shown in FIG. 2a. In one embodiment, beam 14 is modulated or
chopped by circuitry shown schematically in FIG. 4. Beam 14 has an
initial height H.sub.0 and an initial width W.sub.0 determined by
the width of the P-N junction (not shown) of semiconductor laser 10
and by shaping lens 12 and 13.
In one embodiment, the width of the P-N junction is approximately
40 microns and beam 14 as it emerges from shaping lens 13 has a
width of 2.0 mm and a height of 0.4 mm. Laser 10 and beam shaping
lenses 12 and 13 are positioned relative to planar dielectric
mirrors M.sub.1 and M.sub.2 so that the dielectric surfaces S.sub.1
and S.sub.2 of mirrors M.sub.1 and M.sub.2 respectively are
parallel to each other and perpendicular to the plane P through the
center of beam 14 as shown in FIG. 2b. The reflecting surfaces
S.sub.1 and S.sub.2 of mirrors M.sub.1 and M.sub.2 have a length
L.sub.1 and L.sub.2, respectively, and a height h as indicated by
arrows L.sub.1 and L.sub.2 in FIG. 2a and h in FIG. 2c.
In FIG. 2a the solid line emerging from laser 10 denotes the center
of beam 14. Light at the center of beam 14 strikes surface S.sub.1
at P.sub.1 at an angle of incidence .alpha.<90.degree. and is
reflected to point P.sub.2 on surfaces S.sub.2 and in general is
reflected back from P.sub.k to P.sub.k+l where k=1, . . . , 6 thus
creating the zig-zag pattern shown in FIG. 2a. The light reflected
from point P.sub.7 is received by beam stop 20 which contains a
photocell (not shown).
In FIG. 2a the area between each pair of adjacent parallel lines
indicated by long dashes represents a segment of light beam 14
traveling towards surface S.sub.1 and the area between each pair of
adjacent parallel lines indicated by short dashes represents a
segment of beam 14 traveling from surface S.sub.1 towards surface
S.sub.2. The separation S between surfaces S.sub.1 and S.sub.2 and
the angle of incidence .alpha. are chosen so that the distance
between P.sub.1 and P.sub.3 is the width W.sub.0 of beam 14. Since
surfaces S.sub.1 and S.sub.2 are parallel it follows that the
distance between P.sub.k and P.sub.k+2 is W.sub.0 for k=2,3,4,5.
Note that for this selection of parameters S and .alpha. the
zig-zag path of beam 14 creates a sheet of light that covers the
entire area of the trapezoid defined by the end points 15 and 16 of
surface S.sub.1 and points 17 and 18 of S.sub.2. In fact this
entire area is covered both by light traveling from surface S.sub.1
to surface S.sub.2 and by light traveling from surface S.sub.2 to
surface S.sub.1.
FIG. 2d shows an alternate selection of separation S between
surfaces S.sub.1 and S.sub.2 and an alternate selection of the
angle of incidence .alpha. wherein the distance between the first
and second points A.sub.1 and A.sub.3 where the center of beam 14
strikes surface S.sub.1 is greater than the width W.sub.0 of beam
14. For this selection of parameters the zig-zag pattern (light
net) created by the reflections of beam 14 does not cover the
entire area of the trapezoid defined by points 25, 26, 27 and 28 of
reflecting surfaces S.sub.1 and S.sub.2 respectively. The shaded
triangles denote those areas not covered by the beam.
A side view of reflecting surfaces S.sub.1 and S.sub.2 is shown in
FIG. 2c. Collector mirrors 21 and 22 collect light scattered from
particles which fall through the light net generated by beam 14
between surfaces S.sub.1 and S.sub.2. These collector mirrors are
typically parabolic in shape and focus the scattered light on
photodiodes PD.sub.1 and PD.sub.2, respectively. The height, h, of
mirrors M.sub.1 and M.sub.2 is small relative to the dimensions of
collector mirrors 21 and 22 since mirrors 21 and 22 can only
collect light which is reflected out of the plane of the light
sheet generated by laser beam 14 and which also "clears" mirrors
M.sub.1 and M.sub.2 and "overhangs" O.sub.1, O.sub.2, O.sub.3 and
O.sub.4. "Overhangs" O.sub.1, O.sub.2, O.sub.3 and O.sub.4 prevent
dust from settling on surfaces S.sub.1 and S.sub.2 and, being
opaque, prevent light scattered by imperfections in surface S.sub.1
and S.sub.2 from reaching collector mirrors 21 and 22.
Note that the side view of the light net created by the reflection
of beam 14 from surfaces S.sub.1 and S.sub.2 is shown schematically
as merely a line. Absent compensating measures, the thickness of
beam 14 tends to gradually increase as the beam propagates due to
beam divergence. This in turn reduces the power density of the
beam. This gradual increase in beam thickness is not shown in FIG.
2c for the sake of simplicity.
Similarly, without compensation for divergence, the width of beam
14 also increases with path length but this is not shown in FIGS.
2a and 2d for simplicity. The divergence of the width of beam 14 is
generally much smaller than the divergence of the height of the
beam since H.sub.0 is less than W.sub.0.
Dielectric mirrors M.sub.1 and M.sub.2 are typically made of
alternating high and low index coatings on a glass or quartz
substrate. An appropriate mirror is a Melles Griot .lambda./30
mirror with a proprietary MAX-R coating, such as part number/237,
having minimum reflectance of 99.3%. Hence, beam intensity
decreases only gradually as the number of reflections of beam 14
between surfaces S.sub.1 and S.sub.2 increases. Nevertheless, when
employing a semiconductor laser with this invention, commercial
models of which are currently limited in power, e.g. typically less
than 20 milliwatts, surface S.sub.1 and S.sub.2 are separated by a
distance S which is typically larger than L.sub.1 and L.sub.2 in
order to reduce the number of reflections since each reflection
reduces the power density of the reflected beam.
It should be noted with respect to both FIG. 2a and FIG. 2d,
however, that typically more than the six or seven reflections
between surfaces S.sub.1 and S.sub.2 will be employed but a small
number of reflections is shown in the figures for the sake of
simplicity. For example, with S=150 mm, W.sub.o =1 mm, 20
reflections would cover an area approximately 150 mm by 20 mm.
Beam stop 20, which comprises a photocell (not shown in FIG. 2a) in
a housing, terminates beam 14 and monitors the power of the laser
beam incident on the photocell. This monitoring is accomplished
through conventional circuitry (not shown) and serves to indicate
when the system components are misaligned. If the laser beam is
misaligned, the power of the beam incident on the photocell in beam
stop 20 decreases below a preselected level, and conventional
circuitry (not shown) causes a signal to be emitted from beam stop
20. This signal may actuate a visual signal device such as a light
or may be provided to a microprocessor (not shown).
FIG. 2e shows a partial diagram of one embodiment of collector
mirror 21 and photodiode PD.sub.1. The lower portion of the
diagram, which is the mirror image of the top portion, is not shown
for the sake of clarity. Mirror 21 is parabolic in shape. The focus
of the parabola is 2 cm from the vertex V. The center of photodiode
PD.sub.1, which is 0.5 cm in width (vertical dimension in FIG. 2e),
is located at the origin (O,O) of the coordinate system and is 2.6
cm from vertex V. The distance between mirror M.sub.1 and
photodiode PD.sub.1 is 0.2 cm and mirror M.sub.1 is separated from
mirror M.sub.2 by 10 cm. Region 21a between parabolic mirror 21 and
flat surface 21b contains glass having an index of refraction of
1.5 which refracts the rays of scattered light at 15.degree. shown
in FIG. 2e toward the horizontal. (Surface 21b is the front surface
of the glass). The use of such a glass having an index of
refraction greater than 1 increases the angle of acceptance,
.theta..sub.a, which is the largest angle through which light can
be scattered from a particle in beam 14 in front of mirror M1 and
still be reflected to photodiode PD.sub.1 via lens 21.
Table 1 in FIG. 2e shows the acceptance angle .theta..sub.a (in
degrees) as a function of x, where x is the distance of a particle
in beam 14 in front of mirror M.sub.1. As shown in Table 1, the
minimum angle of acceptance for particles at least 1 cm in front of
mirror M.sub.1 is 15 degrees.
FIG. 3a shows scatter cross-section for spherical particles as a
function of angle and particle size for monochromatic light having
a wavelength of 6328 .ANG. (from an HeNe laser) incident on the
particle.
The abscissa in FIG. 3a is labeled in degrees and each abscissa
.theta. represents the solid region between the right circular cone
having angle .theta. and the right circular cone having angle
.theta.+5.degree. as shown in FIG. 3b. The ordinates are measured
in cm.sup.2.
Curves A, B, C, D, E, F, G, H, and I are scatter cross-section
curves for particles having a physical diameter of 0.2 .mu.m, 0.3
.mu.m, 0.4 .mu.m, 0.5 .mu.m, 1.0 .mu.m, 1.5 .mu.m, 2.0 .mu.m, 2.5
.mu.m, and 3.0 .mu.m respectively. Due to interference effects,
each particle has an apparatus cross section which is different
from its physical cross section. The scattering cross section shown
in FIG. 3a is the apparent cross section. An AlGaAs laser diode, as
is used in the preferred embodiment, produces light having a wave
length of 8200 .ANG.. The intensity of scattered light is somewhat
less in this case, but the angular dependence is approximately the
same as that shown in Curves A through I in FIG. 3a. Note that the
most intense scatter is in the forward direction, for example curve
A at approximately 25.degree.. For this reason, the collector lens
system shown in FIG. 2c is used with the lenses located
approximately perpendicular to the direction of travel of laser
beam 14 in order to collect forward scatter. A system with
collector lenses located on the sides of the laser net generated by
laser beam 14 and generally parallel to the directions of travel of
laser beam 14 would be operable but would be much less
efficient.
In one embodiment, the light emitted from laser diode 10 is
electronically chopped (pulsed) in a conventional way by connecting
an AC current source 30 (shown schematically in FIG. 4) to laser
diode 10. The output signal of photocell 31 in beam stop 20 (shown
in FIG. 2a) is fed back to power source 30 of laser diode 10 in
order to maintain a selected constant laser power output. The
purpose of chopping the beam is to produce a particle detection
system that operates in the presence of daylight or light from
other nonmodulated light sources. This greatly improves
signal-to-noise ratio since the detector circuit described below
looks for signals at the chopping frequency rather than at DC. In
one embodiment the frequency of the alternating current source is 3
megahertz and it is preferred to use a frequency sufficiently high
so that the laser beam has at least 10 on-cycles during the time it
takes a particle to fall under the influence of gravity vertically
through the light net generated by beam 14. For example, if it is
assumed that a particle falls (under the influence of gravity)
vertically downward through a beam having a thickness H=0.03 cm at
a velocity of 1500 cm/second (which corresponds to a particle
falling from rest in a vacuum through a distance of approximately
1.15 meters) then ten cycles must occur in 1/50,000 seconds which
corresponds to a frequency of 500 kHz. Since beam 14 is chopped,
the scattered light that is received by photodiodes PD.sub.1 and
PD.sub.2 as a particle falls through the light net generated by
beam 14 is also chopped. The chopped scattered light sensed by
photodiode PD.sub.1 is amplified by amplifier 34a. Amplifier 34a is
a low noise operational amplifier, for example, amplifier LT1037C
made by Linear Technology. Alternately, amplifier 34a can be made
of discrete components and a low noise FET, thereby providing an
even better signal to noise ratio. It is preferable to mount
amplifier 34a within 2 cm of photodiode PD.sub.1 in order to
minimize noise pick-up in the connections. A second gain stage 34b
is mounted in a separate housing indicated schematically by the
dotted line in FIG. 4. A typical output signal of amplifier 34b is
shown (not to scale) by the solid line in FIG. 5a. The dotted line
connecting the positive peaks of the solid line in FIG. 5a is the
"positive envelope" of the signal. The output signal of amplifier
34b is sent to mixer 35 which may be, for example, part no.
XR-2208, an analog multiplier made by Exar. Mixer 35 also receives
the 3 megahertz signal from alternating current source 30. The
output of mixer 35 (shown in FIG. 5b) is the positive envelope of
the output signal of amplifier 34b. The output signal of mixer 35
is amplified by amplifier 36, for example, part no. LT1055 from
Linear Technology, and then provided to peak detector 37 whose
output signal represents the peak magnitude of the envelope shown
in FIG. 6b as amplified by amplifier 36. The output of peak
detector 37 is provided to A/D converter 38 which provides a
corresponding digital signal to microprocessor 39.
In this embodiment the output signal of peak detector 37 is
multiplexed with the output signal of peak detector 37a. Peak
detector 37a is the peak detector associated with photodiode
PD.sub.2. The remaining circuitry associated with photodiode
PD.sub.2 is the same as that associated with photodiode PD.sub.1
and is not shown in FIG. 4 for the sake of simplicity.
The peak detectors 37b, 37c, and 37d in FIG. 4 represent peak
detectors for photodiodes, not shown, which are used in those
embodiments where two or more systems identical to the one shown in
FIG. 2a are ganged together, i.e., placed in close physical
proximity, in order to provide particle detection for a larger
area.
In typical application the computer calculates the particle flux
density, that is, the number of particles which pass through beam
14 is a selected time interval. Microprocessor 39 compares the
signals it receives from peak detectors 37 and 37a. When both
signals are in excess of a preselected threshold value,
microprocessor 39 determines that a particle has fallen through the
light net generated by beam 14. The peak detectors provide signals
which are held for a selected time period, for example, a
millisecond, so that only the largest particle that scatters light
in a selected time period is counted. This is acceptable in
application since most counts are very low, for example in a clean
room typically less than one particle per second is counted. The
microprocessor keeps this count and tracks the time, thereby
enabling it to calculate the particle flux density and to output
the result on a display (not shown) or through an interface bus
(not shown) to an external computer. The microprocessor can also
estimate particle size based on the gain of circuits 34a, 34b, 35,
and 36 and the detected signal strength provided by the A/D
converter. Typically the output of the microprocessor can also be
used to trigger the closing of an interlock relay (not shown) to
terminate operation of processing equipment if the detected
particle flux exceeds a preset critical level.
In this embodiment of the invention shown in FIG. 2d, particles
which fall through the shaded triangles do not, of course, scatter
light to collectors 21 and 22. However if it is assumed that the
particles which fall through the region bounded by mirrors S.sub.1
and S.sub.2 and the light beam are uniformly distributed, then one
may estimate the total particulate count by multiplying the actual
particulate count generated by particles falling through the area
actually covered by the zig-zag path of light beam 14 by the total
area between surfaces S.sub.1 and S.sub.2 divided by the area
actually covered by the zig-zag path of light beam 14.
Since one principle of operation of this system is the detection of
light scattered by a particle falling through the beam, it is
critical to maintain an acceptable power density over the entire
path of the beam. The power density of the beam, which is defined
as the beam power divided by the cross-sectional area of the beam,
drops with the distance the beam is propagated due to beam
divergence.
FIG. 6 shows the vertical beam divergence for a collimated beam
having an initial height H.sub.0 and an initial width W.sub.0
(shown in FIG. 2b) as a function of the distance x that the beam
has propagated. As shown in FIG. 6, after propagating a distance x
the beam thickness H is given by
Moreover, .theta.=.lambda./.pi.H.sub.0 where .lambda. is the
wavelength of the monochromatic laser beam. Therefore
Since .theta. is a small angle
A similar analysis shows that the width of the beam after
propagating a distance x is given by
Thus, the power density of the beam after the beam propagates a
distance x is given by
Assuming that H.sub.0 is much less than W.sub.0, the critical
separation (where the power density is approximately one-half the
original power density) is given by
For example, for .lambda.=1 micron and H.sub.0 =3.times.10.sup.-2
cm, the critical separation occurs where x=15 cm.
If the beam is reflected from surfaces S.sub.1 and S.sub.2 a total
of n times the power density becomes
where R is the reflectivity (typically about 99.5% for the
dielectric mirror surfaces S.sub.1 and S.sub.2).
The power of the scattered light received by detector PD.sub.1
given by
where .eta. is the detector collection efficiency and .sigma. is
the particle scattering cross-section, (i.e., the cross-sectional
area of the particle perpendicular to the beam). Noise limits the
sensitivity of the detector. Noise power is defined by
where N is the Noise Equivalent Power (NEP) of the detector
PD.sub.1 and B is the band width of the electronic circuit 34a,
34b, 35, and 36. Noise Equivalent Power is a standard measure of
the noise power in a frequency bandwidth for a photodiode.
The minimum detectable cross section of a particle is found by
equating the power received by the detector to the noise power and
solving for .rho., the particle scattering cross section, which
yields:
It is desirable to maximize .rho..sub.min as a function of H.sub.0.
The optimal value of H.sub.0 is given by solving
which yields
For example, for a wavelength of 820 nm (AlGaAs laser) and a
propagation distance of 50 cm, H.sub.0, optimal=0.036 cm. For
values of H.sub.0 =0.036 cm, W.sub.0 =0.1 cm, B=200 kHz,
N=5.times.10.sup.-13 watts/H.sub.z.sup.1/2, .eta.=0.5, P.sub.0 =10
milliwatts, .lambda.=820 nm, .rho..sub.min =3.7 10.sup.-10
cm.sup.2. For this .theta..sub.min a 30-degree detector can easily
detect a 0.3 micron-diameter particle. A detector comprising a
collector lens 21 and a photodiode PD.sub.1 is a thirty degree
detector if the photodiode receives rays making an angle up to
thirty degrees with the plane of the light net generated by beam
14. FIG. 7 shows the integrated scatter cross section versus angle
(0.degree. to 10.degree. excluded) for curves, B, C, and D of FIG.
3a. The abscissa .theta. (in degrees) in FIG. 7a represents the
region between the right circular cone having an angle of
10.degree. (with the vertical) and the right circular cone having
an angle of .theta. as shown in FIG. 7b. The power, P.sub..theta.,
of light scattered into this region is given by P.sub..theta.
=I.sub.o .rho..sub..theta. where I.sub.o is the incident power/unit
area and .rho..sub..theta. is the integrated scatter cross section
corresponding to .theta. (FIG. 7a).
As the above analysis shows, the power density decreases with the
propagation distance of beam 14 due to divergence.
Beam divergence can be compensated for by, instead of collimating
the beam using lenses 12 and 13 as explained above, bringing the
beam to a focus that exactly compensates for the divergence angle
.theta. shown in FIG. 6. This is a technique that is well known in
the art, and is discussed, for example, in Melles Griot Optics
Guide 3, page 349, 1985. FIG. 8 shows in more detail one preferred
embodiment for compensating for this divergence. Laser diode 10 is
typically packaged so that light emerging from the p/n junction of
laser 10 passes through glass 10a. The thickness T.sub.1 of glass
10a affects (by refraction) the dimensions of the beam which
emerges from glass 10a and may vary from manufacturer to
manufacturer. The focusing qualities of lenses 41 and 42 are
selected based on a beam which has passed through a selected
thickness T of glass greater than the varying thicknesses for plate
10a typically employed by different manufacturers. Thus, by
inserting glass plate 40 having thickness T.sub.2 where T.sub.2 is
selected so that T.sub.1 +T.sub.2 =T, the remainder of the optical
system may remain unchanged when different laser diodes employing
different glass thickness T.sub.1 are used from time to time.
The beam which emerges from glass plate 40 passes through
cylindrical lens 41 which focuses the beam in the vertical
dimension indicated by the arrow Ho in FIG. 2b. The radius of
curvature of lens 40 is selected to exactly compensate for the
vertical beam divergence angle .theta. shown in FIG. 6. The beam
emerging from cylindrical lens 41 then passes through cylindrical
lens 42 which has a radius of curvature selected to exactly
compensate for horizontal beam divergence (divergence in the width
of the beam). Beam 14 emerging from lens 42 thus maintains a
constant thickness and a constant width as it propagates between
mirrors M.sub.1 and M.sub.2.
Alternatively, divergence in the thickness H of beam 14 can be
corrected by slightly curving the mirror surfaces S.sub.1 and
S.sub.2. The use of cylindrical lenses 41 and 42 described above is
preferable to this second method since curved mirrors are expensive
and difficult to implement.
A third method of collimation and divergence correction is to use a
gradient index lens. Gradient index lenses are available from
Melles Griot in the 06LGT product line in consumer specified
legnth. A gradient index lens is a glass rod with an index of
refraction that varies with diameter. A focusing action results as
light propagates through the rod. The focal length of the gradient
index lens is determined by the length of the glass rod. FIG. 9
shows one embodiment of the optical means used to compensate for
beam divergence employing a gradient index lens. The elements in
FIG. 9 that are the same as in FIG. 8 bear the same numerical
labels. In FIG. 9, the beam emerging from flat glass plate 40 is
received by cylindrical lens 50 selected so that the beam emerging
from lens 50 has a desired ratio of height to width. The beam from
lens 50 is received by gradient index lens 51 which collimates the
beam along the horizontal (width) axis and compensates for beam
divergence angle .theta. (FIG. 6) along the vertical thickness
axis, so that the light net generated by beam 14 has constant
thickness.
With reference to FIGS. 10, 11, 12a and 12b, a compact direct view
sensor is illustrated. The direct view sensor assembly includes a
light source 60, such as a laser cartridge, having lenses that
provide a collimated laser beam 62. The beam is directed to a first
reflecting mirror 64 at an angle of about 15.degree. relative to
the planar face of the mirror 64. The beam is reflected to a second
mirror 66 that has a face substantially parallel to that of the
first mirror 64 so that a light sheet or light net is produced in
the narrow gap between the mirrors. The reflected beam passes over
the first mirror 64 to impinge on a photodiode 68. The photodiode
indicates the intensity of the light beam that is received to
ensure that the laser beam and the optical system of the sensor
assembly are operating properly. The beam is reflected from the
photodiode into a beam stop cavity 70 which prevents stray light
from returning to the optical system, and provides a safety
feature.
As illustrated in FIG. 11, photocells 72 and 74 are positioned
close to the mirrors 64 and 66 to detect the traversal of particles
through the light sheet. The photocells generate signals
representative of the incidence of particles passing in the gap
between the mirrors. The signals are amplified, peak detected and
processed by a microprocessor that computes the particle flux
density, as described previously. A visual display is obtained by
means of a display monitor that is coupled to the microprocessor.
Glass filters 74 and 76 are provided with the photocells to block
spurious light of different wavelengths than that of the laser
beam. The mirrors have front pieces 80 and 82 which are made with
dielectric stacks, formed of a substrate with a MAX-R (trademark of
Melles Griot) coating that passes a wavelength of about 780
nanometers. The front pieces are attached to cover glasses 84 and
86 that are seated over the faces of the respective mirrors.
The sensor assembly is positioned in a frame 88, and support
brackets 90, one being illustrated in FIG. 12, enclose the ends of
the frame to form a housing or enclosure for the sensor
assembly.
An alternative embodiment of a sensor assembly is depicted in FIGS.
13 and 14. The sensor assembly, which is useful for particle
detection of hot or corrosive gasses or fluids, is made with a pipe
section 89 that is disposed in a substantially transverse direction
between two reflecting mirrors 91 and 92. The pipe section 89 has a
narrow central portion 94 and flared flange portions 96a, 96b which
enable coupling the pipe section to standard diameter pipes through
which the gas or fluid under detection is passed. Glass windows 98
and 100 are provided between the mirrors and the central cavity of
the pipe section. The glass windows protect the optical system from
corrosion and heat. The windows are made of a heat resistant, low
temperature coefficient glass, such as fused silica. The glass is
polished flat on the front and back surfaces. The mirrors are
assembled to the windows, which are mounted onto the pipe using
fluorocarbon O-rings that are able to withstand temperatures up to
200.degree. C. The mirrors are aligned by tightening two sets of
three bolts 102 and 104 that clamp the windows to the pipe, while
the O-rings provide the desired play.
When operating in high temperature environments, the housing for
the laser 106 and the beam stop structure 108, which includes the
photodiode 109, are cooled by circulating cooling fluid or water
through channels or pipes positioned in the housing. As previously
described, a preamplifier 110 is provided to amplify the intensity
of the laser beam.
With the small gap between the mirrors, as embodied in the
alternative implementations of FIGS. 10-14, a compact structure
that provides improved signal resolution is made possible. The
compact structures are made with a reduced number of parts and are
less costly to manufacture.
The scope of the invention is not limited to the specific
arrangements, materials and parameters disclosed herein. For
example, more than two mirrors may be used to generate the light
net, and the optical system may be modified to provide a desired
path for the light beam. Also, the pipe section described with
reference to FIGS. 13 and 14 may be cylindrical, a rectangular
channel, or any configuration affording the compact narrow gap
between the reflecting mirrors.
* * * * *