U.S. patent application number 10/927671 was filed with the patent office on 2005-02-03 for particle detection system.
Invention is credited to Brady, James, DeFreez, Richard K., Girvin, Kenneth L..
Application Number | 20050024641 10/927671 |
Document ID | / |
Family ID | 32908263 |
Filed Date | 2005-02-03 |
United States Patent
Application |
20050024641 |
Kind Code |
A1 |
DeFreez, Richard K. ; et
al. |
February 3, 2005 |
Particle detection system
Abstract
A particle detection system exhibits an increased ability to
detect the presence of submicron diameter particles and to
distinguish between noise and pulse output signals generated by
small diameter particles on which a light beam is incident. This
increased ability results from the incorporation of a light
reflector, a pair of detector elements that detect correlated
portions of the light beam that have been scattered in multiple
directions, and a coincidence circuit that determines whether each
detector element in the pair concurrently generates a pulse output
signal exceeding a predetermined threshold. Sample particles are
counted only when both detector elements concurrently detect
scattered light components.
Inventors: |
DeFreez, Richard K.;
(Azalea, OR) ; Brady, James; (Ashland, OR)
; Girvin, Kenneth L.; (Grants Pass, OR) |
Correspondence
Address: |
STOEL RIVES LLP
900 SW FIFTH AVENUE
SUITE 2600
PORTLAND
OR
97204
US
|
Family ID: |
32908263 |
Appl. No.: |
10/927671 |
Filed: |
August 27, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10927671 |
Aug 27, 2004 |
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10407650 |
Apr 4, 2003 |
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6784990 |
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Current U.S.
Class: |
356/343 |
Current CPC
Class: |
G01N 2015/1402 20130101;
G01N 15/1459 20130101; G01N 15/1429 20130101; G01N 21/53
20130101 |
Class at
Publication: |
356/343 |
International
Class: |
G01N 021/00 |
Claims
1. In a particle detection system for detecting sample particles
entrained in a fluid stream, the particle detection system having a
view volume that has an effective center and in which a light beam
propagating along a light propagation path along an optical axis
transversely intersects the fluid stream such that entrained
particles upon which the light beam is incident scatter first and
second correlated light components, the first light components
scattered in first directions for propagation through a light
collection lens system and incidence on a detector that produces
signals representing the intensities of the first light components,
and the second light components scattered in second directions that
are different from the first directions, comprising: a light
reflector positioned relative to the effective center of the view
volume to receive the second light components, the light reflector
having light directing properties that cause reflections of the
second light components in the form of corresponding inverted
second light components that propagate through the view volume and
the light collection lens system; an array of first and second
detector elements corresponding to spatial locations in the view
volume of the entrained particles causing scattering of the first
and second light components, the first detector elements being
members of a first set of detector elements in the array and
receiving the first light components and the second detector
elements being members of a second set of detector elements in the
array and receiving the corresponding inverted second light
components, different pairs of the first and second detector
elements in the respective first and second sets corresponding to
different spatial locations along the light propagation path within
the view volume and each of the different pairs producing detector
pulse output signals representing the intensities of the first
light component and its corresponding inverted second light
component the pair received; and a signal processing system that
responds to the detector pulse output signals by combining them to
increase a detection system signal-to-noise ratio and thereby
enhancing the detectability of the first and second light
components to record presences of their corresponding particles
entrained in the fluid stream.
2. The particle detection system of claim 1, in which the light
reflector includes a curved mirror.
3. The particle detection system of claim 2, in which curved mirror
is of elliptical, aspherical, or spherical shape.
4. The particle detection system of claim 2, in which the curved
mirror is of spherical or aspherical shape and has an optical
center of curvature, the effective center of the view volume and
center of curvature of the curved mirror being aligned with each
other.
5. The particle detection system of claim 2, in which the curved
mirror is of spherical shape and has a center of curvature and in
which the light collection lens system has a collection system
axis, the curved mirror and the light collection lens system
positioned on opposite sides of the view volume with the center of
curvature of the curved mirror centered on the collection system
axis.
6. The particle detection system of claim 2, in which the curved
mirror is of spherical shape and has a center of curvature and in
which the light collection lens system has a collection system
axis, the curved mirror and the light collection lens system
positioned on opposite sides of the view volume with the center of
curvature of the curved mirror centered on the optical axis.
7. The particle detection system of claim 2, in which the light
collection lens system has a collection system axis, and in which
each of the different pairs of the first and second detector
elements represents an opposing pair of detectors that are
positioned at equal distances from the collection system axis to
view a common spatial location of the view volume by receiving
concurrently the first and second light components propagating
through the common spatial location.
8. The particle detection system of claim 1, in which the view
volume is located within a flow chamber.
9. The particle detection system of claim 1, in which the signal
processing system includes a comparator with a preset threshold and
in which the combining of the detector pulse output signals
includes determining whether signals corresponding to the pulse
output signals produced by each of the different pairs of the first
and second detector elements exceed the preset threshold.
10. The particle detection system of claim 1, in which the signal
processing system includes multiple comparators with preset
thresholds and in which the combining of the detector pulse output
signals includes determining whether signals corresponding to the
pulse output signals produced by each of the different pairs of the
first and second detector elements, and exceeding the preset
thresholds, are coincident in time.
11. The particle detection system of claim 10, in which detector
pulse output signals that lack coincidence in time are rejected
such that they are not processed by the signal processing
system.
12. The particle detection system of claim 1, in which the signal
processing system includes a pulse summation amplifier that sums
coincident pulse output signals produced by each of the different
pairs of the first and second detector elements.
Description
RELATED APPLICATIONS
[0001] This application is a division of U.S. patent application
Ser. No. 10/407,650, filed Apr. 4, 2003.
TECHNICAL FIELD
[0002] The present invention relates to optical particle detection
and, in particular, to a particle detection system with increased
sensitivity in the detection of submicron diameter particles.
BACKGROUND OF THE INVENTION
[0003] Contamination control, including particle monitoring, plays
a critical role in the manufacturing processes of several
industries. These industries require clean rooms or clean zones
with active air filtration and require the supply of clean raw
materials such as process gases, deionized water, chemicals, and
substrates. In the pharmaceutical industry, the Food and Drug
Administration requires particle monitoring because of the
correlation between detected particles in an aseptic environment
and viable particles that contaminate the product being produced.
Semiconductor fabrication companies require particle monitoring as
an active part of quality control. As integrated circuits become
more compact, line widths decrease, thus reducing the size of
particles that can cause quality problems. Accordingly, it is
important to detect and accurately measure submicron particles of
ever-decreasing sizes and numbers per volumetric unit.
[0004] To perform particle monitoring, currently commercially
available submicron particle detection systems use optical
detection techniques to determine the presence, size, and number of
particles in a volumetric unit. The foundation of this technology
is optical scattering of a light beam and detection of the optical
signal after it has been scattered by a sample particle. The
standard particle detection approach, which was developed during
the late 1980s, entails intersecting, in a region referred to as a
"view volume," a light beam and a sample stream containing sample
particles. Light scattered from the view volume is collected with
optics and focused onto a detection system that collects the volume
of light and projects it onto one or more detector elements. The
ability of a particle detection system to detect small particles
depends on its ability to distinguish between noise and pulse
output signals generated from light scattered by submicron sample
particles.
[0005] What is needed, therefore, is a particle detection system
having high submicron particle detection sensitivity.
SUMMARY
[0006] An object of the invention is, therefore, to provide a
particle detection method and system characterized by increased
submicron particle detection sensitivity and accurate particle size
determination.
[0007] The particle detection system of the present invention
includes a flow chamber within which a light beam and a fluid
stream containing sample particles intersect to form a view volume.
The incidence of a sample particle on the light beam causes
portions of the light beam to scatter from the view volume in the
form of first and second correlated scattered light components. The
first correlated scattered light component exits the view volume in
a first direction, is collected and focused by a light collection
lens system, and is detected by a first detector element of a pair
of detector elements located in an array of detector elements. The
second correlated scattered light component exits the view volume
in a second direction and is incident upon a light reflector. The
light reflector reflects in an inverted state the second correlated
scattered light component and focuses it into the view volume. The
second correlated scattered light component then passes through the
view volume, is collected and focused by the light collection lens
system, and is detected by a second detector element of the pair of
detector elements.
[0008] Each detector element in the pair of detector elements
detects the incidence of light and generates a pulse output signal,
the magnitude of which depends on the intensity of the incident
scattered light component. A signal processing system performs
analog or digital signal processing of only those pulse output
signals that are temporally and spatially coincident such that both
of the first and second detector elements of the pair of detector
elements concurrently generate pulse output signals. If each of the
pulse output signals concurrently crosses its associated threshold,
the signal processing system filters the pulse output signals to
remove noise and amplifies the signals to generate a final pulse
output signal indicating the presence and size of the sample
particle.
[0009] Signal enhancement results from the required temporal and
spatial coincidence of pulse output signals corresponding to the
same sample particle. Because sample particles are counted only
when both detector elements of a pair concurrently detect a
scattered light component and when the resultant pulse output
signals exceed a predetermined threshold, randomly occurring noise
pulses or excursions are unlikely to concurrently contact both of
the detector elements in the pair. Specifically, the probability
that two pulse output signals will concurrently exceed the
predetermined threshold is equal to the square of the probability
that an individual pulse output signal will exceed the threshold.
The coincidence function permits the use of a lower threshold for a
given false count rate because most noise is random and will not
concurrently trigger both detector elements in the pair. Use of a
lower threshold facilitates the detection of smaller sample
particles.
[0010] The particle detection system also preferably includes a
noise detection and cancellation system that prevents noise from
triggering a coincidence event. The preferred noise detection and
cancellation system includes a noise detector that is positioned to
monitor only the laser beam and a cancellation unit that removes
(i.e., by subtraction or division) the signal generated by the
noise detector from the signal generated by each detector
element.
[0011] The particle detection system of the present invention has
an increased ability to distinguish between noise and low-amplitude
pulse output signals caused by small diameter particles. The
required temporal and spatial coincidence of pulse output signals
results in signal enhancement. Because sample particles are counted
only when both symmetrically opposed detector elements concurrently
detect scattered light correlated components having a pulse output
signal that exceeds a predetermined threshold, the incidence of
randomly occurring noise pulses or excursions causing a false
signal is significantly decreased. Specifically, the probability
that two pulse output signals will concurrently exceed the
predetermined threshold is equal to the square of the probability
that an individual pulse output signal will exceed the threshold.
Consequently the threshold for a given false count rate may be
lowered by more than a factor of the square root of two while
maintaining the desired overall false count rate. Thus the
coincidence function allows the use of a lower threshold setting
without increasing the incidence of false particle signals, since
most noise is random and is unlikely to concurrently trigger both
detector elements of a symmetrically opposed pair. The use of a
lower threshold facilitates more accurate detection of smaller
diameter particles.
[0012] Additional aspects and advantages will be apparent from the
following detailed description of preferred embodiments, which
proceeds with reference to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 is a plan view of a preferred embodiment of the
particle detection system of the present invention.
[0014] FIG. 2 is a schematic diagram of a preferred signal
processing system for use with the particle detection system of the
present invention.
[0015] FIG. 3 is a schematic diagram of an alternative signal
processing system for use with the particle detection system of the
present invention.
[0016] FIG. 4 is a schematic diagram of an alternative signal
summation scheme for use with the particle detection system of the
present invention.
[0017] FIG. 5 is a graphical curve from which the probability of
generating a minimum specified number of false predetermined
particle counts during a predetermined time interval can be
determined.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0018] The particle detection system of the present invention has
an increased ability to distinguish between noise and pulse output
signals generated by small particles incident upon a light beam.
This increased ability results from the incorporation of a light
reflector, a pair of detector elements that detect correlated
portions of a light beam scattered in multiple directions by a
particle, and a coincidence circuit that determines whether the
detector elements in the pair concurrently generate pulse output
signals exceeding a predetermined threshold. If both detector
elements of the pair concurrently generate pulse output signals,
there is a high probability that the signals were caused by the
incidence of a sample particle on the light beam rather than by
noise variations in the particle detection system. The ability of
the particle detection system of the present invention to
distinguish low-amplitude pulse output signals from noise enables
the system to detect smaller diameter particles than those
detectable by prior art particle detection systems.
[0019] FIG. 1 shows a preferred embodiment of a particle detection
system 10 configured in accordance with the present invention.
System 10 includes a flow chamber 12 through which a
particle-carrying sample fluid stream 14, such as air, flows in a
flow direction (out of the plane of FIG. 1) and a light beam 18
emitted by a light source 22 propagates in a direction along an
optical axis 20. Light source 22 is preferably a diode laser that
is robust, efficient, and compact. Exemplary preferred light
sources are gas, dye, or solid-state lasers. Fluid stream 14 and
light beam 18 intersect within the effective center of flow chamber
12 in a region called a view volume 26.
[0020] Following its incidence on a particle 28 present in view
volume 26, light beam 18 scatters as components of light initially
propagating in first and second generally opposite directions. In a
preferred embodiment, light components 30 propagate in the first
general direction and light components 34 propagate in the second
general direction, reflect off and are inverted about an optical
axis by a light reflecting optical element 36, and propagate back
into view volume 26. Light components 30 and 34 then propagate in a
direction along a collection system axis 38 through a light
collection lens system 40 that converges light components 30 and 34
on corresponding detector elements of a photodetector array 42.
Because they are scattered by the same particle 28, light
components 30 and 34 are correlated in that their incidence on the
corresponding detector elements produces detector output signals
indicative of the size of particle 28 that scattered light beam
18.
[0021] Second optical element 36 is preferably a light reflecting
optical element, such as a curved light reflector having an outer
surface that is coated with a high-reflectance coating. More
preferably, second optical element 36 is a light reflector in the
form of a curved segment with a curved outer surface that is
spherically, elliptically, or aspherically shaped. Preferably, the
light reflector is a mirror and is positioned opposite light
collection lens system 40 with view volume 26 disposed between
them. Light reflector 36 is preferably centered on collection
system axis 38 such that the center of curvature of light reflector
36 is aligned with the effective center of view volume 26. Light
reflected off second optical element 36 and propagating back
through view volume 26 is inverted about collection system axis
38.
[0022] FIG. 1 is useful in the explanation of the processing of
light components 30 and 34 formed by the incidence of light beam 18
on particle 28. The incidence of light beam 18 upon particle 28
scatters first and second correlated light components 30 and 34
from view volume 26 in, respectively, first and second directions.
First light component 30 is collected by light collection lens
system 40 and focused onto a first detector element 70 of
photodetector array 42. Photodetector array 42 is positioned so
that its light receiving surface 72 is substantially perpendicular
to and the number of detector elements in the linear array is
bisected by collection system axis 38. Particles struck by light
beam 18 at different locations within view volume 26 will cause
points of light to be focused onto photodetector array 42 in
different positions corresponding to the relative locations of the
particles within view volume 26.
[0023] Light collection lens system 40 collects light scattered by
particles within view volume 26 and images the light onto
photodetector array 42. Exemplary light collection lens systems are
commonly known to those with skill in the art; however, a
refractive light collection lens system is preferred.
[0024] Second light component 34 is scattered by particle 28 from
view volume 26 in a second direction initially away from light
collection lens system 40 and is incident upon second optical
element 36, which acts as a light reflector that reflects and
inverts about optical axis 20 second light component 34. As shown
in FIG. 1, second light component 34 returns to view volume 26 in
an inverted state at a location approximately the same distance
from, but on the opposite side of, collection system axis 38 as
that of first light component 30. The inverted second light
component 34 propagates through view volume 26 and light collection
lens system 40, which focuses the inverted second light component
onto a second detector element 76 of photodetector array 42. First
and second detector elements 70 and 76 constitute a related pair of
detector elements of photodetector array 42 such that first
detector element 70 is spatially related to the first light
component 30 scattered by particle 28 and second detector element
76 is spatially related to the inverted second light component 34
scattered by particle 28. Because of the above-described system
component alignment, first and second detector elements 70 and 76
are approximately equidistant from and on opposite sides of
collection system axis 38.
[0025] Photodetector array 42 is preferably a linear array of
photodiode detectors having dimensions that are proportional to the
image dimensions of view volume 26. An array of photodiode
detectors is used for the purpose of detecting coincidence and
thereby reducing noise and false counts. As stated above,
collection system axis 38 divides array 42 into two sets of
detector elements, one that contains first detector element 70 and
another that contains second detector element 76.
[0026] Detector elements 70 and 76 are preferably equidistant from
collection system axis 38. Exemplary detector arrays include an
avalanche photodetector (APD) array, a photomultiplier tube (PMT)
array with an array of anodes, and a photodetector (PD) array. An
exemplary commercially available photodetector array is the Perkin
Elmer Optoelectronics Model C30985E, with 25 detector elements each
measuring 0.3 mm center-to-center.
[0027] First and second detector elements 70 and 76 generate pulse
output signals corresponding to, respectively, first and second
correlated light components 30 and 34. The amplitude of a pulse
output signal is dependent on the size of the particle to which it
corresponds. Signal processing takes place downstream of
photodetector array 42 and converts the pulse output signal into a
voltage that can be measured. Because particle size is directly
related to light amplitude, which is directly related to pulse
output signal amplitude, which is directly related to voltage
magnitude, the size of a particle may be determined by measuring
the voltage corresponding to each pulse output signal. The signal
is preferably amplified before reaching the preamplifier stage,
where inherent electronic noise is added to the signal. Because the
signal has already been amplified, the proportional amount of
electronic noise added at the preamplifier stage is smaller than
what it would have been had the signal not been amplified before
reaching the preamplifier stage.
[0028] As shown in FIG. 2, a preferred signal processing system 78
of the present invention includes a transimpedance amplifier 80
that converts from electric current to voltage the pulse output
signals generated by one of first and second detector elements 70
and 76 in photodetector array 42. Next, background noise is
canceled from the voltage signal at a noise cancellation unit 84
that subtracts or divides a baseline pulse measured by a noise
detector 88. Noise detector 88 is preferably an end-cavity detector
or a detector element that is not used to perform particle
detection. Noise cancellation minimizes the incidence of false
particle signals based on noise common to the light source.
Division-based noise cancellation is preferable to subtractive
noise cancellation because division-based cancellation is more
robust and provides greater tolerance of differences in phase
shift, time delay, and gain. Following noise cancellation, the
pulse output signals are filtered by a filter 90, the shape of
which is preferably similar to the shape of the frequency response
characteristic of the pulse output signal.
[0029] Such a matched filtering approach is described in, for
example, Intro to Statistical Communication Theory by John B.
Thomas. Next, a comparator 92 compares the pulse output signal to a
predetermined threshold that is typically slightly above the
average noise of the system. If the pulse output signal is less
than the threshold, the signal is ignored. If the pulse output
signal is greater than the threshold, the signal is converted to a
digital ON/OFF signal and sent to an AND gate 81. The AND gate 81
operates as follows: if both voltage signals are ON, the pulse
output is ON; if both voltage signals are OFF, the pulse output is
OFF; if only one voltage signal is ON, the pulse output is OFF.
Thus an ON pulse output is generated only when two ON voltage
signals are concurrently received at AND gate 81.
[0030] As shown in FIG. 3, an alternative processing system of the
present invention includes a transimpedance amplifier 80 that
converts from electric current to voltage the pulse output signals
generated by one of first and second detector elements 70 and 76 in
photodetector array 42. Once the pulse output signals have been
converted to voltage values, they are summed at one or more pulse
summation amplifiers 82. As described above, background noise is
canceled from the pulse output signal at a noise cancellation unit
84 that subtracts or divides a baseline noise pulse measured by a
noise detector 88. Following noise cancellation, the pulse output
signals are filtered by a filter 90, the shape of which is
preferably similar to the shape the frequency response
characteristic of the pulse output signal. Finally, a comparator 92
compares the pulse output signal to a predetermined threshold that
is typically slightly above the average noise of the system. If the
pulse output signal is less than the threshold, the signal is
ignored. If the pulse output signal is greater than the threshold,
the signal is processed by a computer (not shown) that measures
voltage and determines particle size therefrom.
[0031] An alternative signal summation scheme, shown in FIG. 4,
entails summing at signal summation amplifiers 82 the pulse output
signals of pairs of adjacent detector elements. This alternative
signal summation scheme is preferably implemented in particle
detection systems detecting large diameter particles for which the
light beam propagating through view volume 26 is greater than the
width of a single detector element in photodetector array 42. The
signal summation scheme of FIG. 4 effectively doubles the width of
each detector element in photodetector array 42, facilitating
accurate detection of pulse output signals generated by large
diameter particles. By summing the signals from adjacent detector
elements, signal amplitude information and resolution are
preserved.
[0032] Many signal processing systems are known to those skilled in
the art. The above-described signal processing systems are merely
exemplary and, as skilled persons know, can be modified to achieve
the objectives of the present invention. Such signal processing
systems are within the scope of the present invention.
[0033] The preferred embodiments of the particle detection systems
of the present invention have an increased ability to distinguish
between noise and low-amplitude pulse output signals caused by
small diameter particles. Signal enhancement results from the
required temporal and spatial coincidence of pulse output signals.
Because sample particles are counted only when both symmetrically
opposed detector elements concurrently detect scattered light
correlated components having a pulse output signal that exceeds a
predetermined threshold, the incidence of randomly occurring noise
pulses or excursions causing a false signal is significantly
decreased. Specifically, the curve shown in FIG. 5 indicates that
the probability that two pulse output signals will concurrently
exceed the predetermined threshold is equal to the square of the
probability that an individual pulse output signal will exceed the
threshold. Consequently the threshold for a given false count rate
may be lowered by more than a factor of the square root of two and
still maintain the desired overall false count rate. Thus the
coincidence function allows the use of a lower threshold setting
without increasing the incidence of false particle signals, since
most noise is random and is unlikely to concurrently trigger both
detector elements of a symmetrically opposed pair. The use of a
lower threshold facilitates more accurate detection of smaller
diameter particles.
[0034] It will be obvious to those having skill in the art that
many changes may be made to the details of the above-described
embodiments without departing from the underlying principles of the
invention. The scope of the present invention should, therefore, be
determined only by the following claims.
* * * * *