WO1990010858A1

WO1990010858A1 - Single particle detector using light scattering techniques

Info

Publication number: WO1990010858A1
Application number: PCT/US1990/001119
Authority: WO
Inventors: James L. Blesener; Stanley L. Kaufman; Peter P. Hairston; Frank D. Dorman
Original assignee: Tsi Incorporated
Priority date: 1989-03-06
Filing date: 1990-03-02
Publication date: 1990-09-20

Abstract

The device (10) utilizes an impactor (17a) to set an upper limit on particle size. Sample fluid is passed through a high density light, with the intersection of the fluid and high density light forming a sensing volume. Particles in the fluid scatter light which is collected and focused by spherical mirrors (41, 42) onto detectors (34, 35). The scattered light causes a signal to be generated which is then compared by coincidence detecting circuitry in order to qualify whether a particle scattered the light.

Description

SINGLE PARTICLE DETECTOR

USING LIGHT SCATTERING TECHNIQUES

Field of the Invention

This invention relates generally to the detection of particles, and more specifically to the detection of single particles using light scattering techniques and coincidence collection of the scattered light.

Background of the Invention Semiconductor fabrication techniques often utilize line widths below 1 micrometer (urn) and layer thickness below 0.1 urn. Airborne contamination in the form of small particles can cause defects in semiconductor devices when coming into contact with the semiconductor during its manufacture. Further, there is a trend toward smaller line widths and layer thicknesses which is continuing. Therefore, the environmental conditions under which semiconductor devices are manufactured are closely monitored in order to control such contamination. While there are multiple techniques for monitoring and sizing particles suspended in air, optical detection of light scattered by the particles (hereafter referred to as "light scattering" for convenience) is often utilized.

One such particle monitoring device is the model 3753 LPC laser particle counter manufactured by TSI, Inc., the assignee of the present invention. This device utilizes a plurality of lenses to collect light scattered by contaminant particles passing through a laser beam. The collected light is focused onto a single detector, thereby generating electrical signals which are subsequently counted. Utilizing this arrangement, the device is capable of measuring particles down to .3 um. As the trend toward smaller fabrication techniques continues, however, instruments capable of smaller particle measurements are required. - 2 -

The foregoing device is further limited in particle size sensing capabilities due to its limited ability to collect the scattered light. Also, the device may measure one particle several times due to recirculation of the 5. airborne particle within the test cavity of the device. Still further, this device may measure the presence of particles when none are actually present due to the device's limited finite electrical noise immunity as well as by measuring ionizing radiation incident on the device's 0 detectors.

Other current light scattering devices suffer similar drawbacks in that the devices collect only a portion o light scattered by the particles being measured. Optimally, all components (i.e., forward, backward an 5 side) of the scattered light should be collected. B increasing the total collected light, the signal to nois ratio of a device may be improved.

As can be appreciated, these prior methods o monitoring "clean room" type environments, such as th 0 environment in which semiconductors are typicall fabricated, have been characterized by sensitive devices. However, as increased device sensitivity is required, th rate of occurrence of signals due to real particle approaches the order of magnitude of the rate of occurrenc 5 of signals caused by ionizing emissions striking th device's opto-electric photo detector. Therefore, suc devices may be unable to qualify whether a signal i generated by scattered light from a particle or whether th signal is generated from ionizing emissions striking th 0 detection device or photo-diode.

Therefore, there is a need for a device which is abl to determine the difference between two such signa generating events in order to include as countable event only those signals generated by light scattered fro 5 particles. There is also a need for a compact particl counting apparatus which provides for increased sensitivit and performance. Summary of the Invention The present invention provides for an improved method and apparatus for determining the concentrations and sizes of fluid borne particles by means of a light scattering device while eliminating spurious counts through use of coincidence detection means. Preferably, a device constructed according to the principles of the present invention includes coincidence determining means which qualify whether light scattered by a particle has struck a plurality of sensor means within a certain period of time. Utilizing coincidence detection reduces, or eliminates, counting ionizing emissions, as particles due to the low probability that ionizing emissions will strike the sensor means at the same moment of time. In a preferred embodiment apparatus constructed according to the principles of the present invention, an optical particle counter takes a sample of fluid and pulls it through a beam of light. The intersection of the fluid and beam of' light form a sensing volume. Particles contained in the fluid sample scatter light as they pass through the beam. The light which is scattered by the particle is related to the particle's size, shape and index of refraction. This scattered light is converted to an electrical signal by a photoelectric device. Refractive, reflective or other optical light collection techniques may be used to enhance the efficiency of converting the scattered light into electrical signals. Preferably, spherical mirrors are utilized to enclose nearly 4 π steradians about the sensing volume in order to collect as much of the scattered light as possible. The photoelectric device causes electrical signals which are a function of the detected particle size.

Therefore, according to one aspect of the invention, there is provided a continuous flow optical particle counter apparatus of the type wherein a sample fluid is passed through a beam of light, the intersection of the fluid and beam of light forming a sensing volume, and wherein particles suspended in the sample fluid scatter light, the particle counter comprising:

(a) a plurality of sensor means for collecting scattered light and generating signals responsive to the scattered light; and

(b) coincidence determining means, cooperatively connected to said sensor means, for comparing said generated signals to one another so as to count only those signals which coincide, whereby only events which trigger a plurality of said sensor means are counted.

While the invention will be described with respect to a preferred embodiment configuration and with respect to particular components used therein, it will be understood that the invention is not to be construed as limited in any manner by either such configuration or components.

These and various other advantages and features which characterize the invention are pointed out with particularity in the claims annexed hereto and forming a part hereof. However, for a better understanding of the invention, its advantages and objectives obtained by its use, reference should be had to the drawing which forms a further part hereof and to the accompanying descriptive matter, in which there is illustrated and described a preferred embodiment of the invention.

Brief Description of the Drawings Referring to the drawings, wherein like numbers represent like parts throughout the several views:

FIGURE 1 is perspective view of an apparatus 10 constructed according to the principles of the present invention;

FIGURE 2 is a cross-section view of the apparatus of Fig. 1 taken through line 2-2 of Fig. 1;

FIGURE 3 is a cross-section view of the apparatus of Fig. 1 taken through line 3-3 of Fig. 1;

FIGURE 4 is a diagram of the functional blocks of the apparatus of Fig. 1 illustrating the coincidence circuitry; FIGURE 5a is a top plan view of a spherical mirror of Fig. 2;

FIGURE 5b is in a side elevational view taken through line 5b-5b of Fig. 5a of a spherical mirror; and FIGURE 6 is an unsealed diagrammatic illustration of the flow path of fluid through a portion of the apparatus of Fig. 1.

Detailed Description of the Invention

The principals of this invention apply particularly well to its application within a particle counting light scattering device. The present invention is compact in design and its many elements interact with each other to make an efficient, reliable measurement instrument.

Light may undergo multiple interactions when it encounters a particle. The five major interactions between particles and light are as follows: (1) Diffraction, when photons of light pass by the particle and are bent toward or away from it; (2) refraction, when photons pass through the particle ' and their path is changed because of the different indices of refraction of the two media; (3) reflection, when photons hit the particle and are reflected away; (4) absorption, when photons hit the particle and are absorbed into it, transforming their energy into heat; and (5) thermal emission, when particles cool and emit photons. See for example, Bohren and Huffman, Absorption and Scattering of Light by Small Particles (1983), for a more thorough discussion of such light interaction.

Light scattering may be considered in two ranges of approximation dependent upon the relationship between the wavelength of light and the particle size. The first range of approximation is known as Rayleigh scattering. This range is appropriate when the wavelength of the light is much greater than the diameter of the particle(s) to be measured. As those skilled in the art will recognize, the particle to be measured is that particle on which the light is incident. In this range of light scattering, the main interactions of the light wave with the particle are diffraction, refraction, reflection, absorption and emission.

The second range is called Geometric scattering. This range applies when the diameter of the particle is much greater than the wavelength of the light source.

Alternatively, ie scattering theory may be used to provide an exact solution to the light scattering phenomenon over the entire range of particle sizes versus wavelength spectrum. However, due to this technique's complexity in the Rayleigh and geometric scattering regions, Mie scattering theory is usually used only when the wavelength of light is approximately equal to the diameter of the particle in test.

As is well known, any combination of the foregoing light phenomenon may be used for particle sensing.

An example of a preferred embodiment of a light scattering device constructed according to the principles of the present invention is illustrated in Fig. 1 as apparatus 10. Referring to Fig. 1, apparatus 10 is illustrated as having a cover 13 forming a top and two sides. Such cover 13 is constructed of sheet metal, although any type of rugged exterior housing may similarly be used as a matter of design choice. Nozzle 11 extends through cover 13 as well as removable cap 12 (described below) . Cover 13 is attached to apparatus 10 via screws 14 although any number of other well known fastening means might be used.

A plurality of light emitting diodes (LED) 15, 16 extend through one end of end plate 60. One LED 15 indicates the on/off status of apparatus 10. The second LED 16 indicates the detection of a particle. Extending from the rear plate (not shown) of apparatus 10 is a connector device (now shown) such as an RJ-ll or RS-232 style connection device for output to a computer or microprocessor (not shown).

In order to better understand the present invention, the coincidence circuitry and other electrical matters of apparatus 10 will now be deferred pending a more detailed description of the flow path of the air sample through device 10 and the device's 10 optical arrangement.

Referring next to Fig. 2, the sample fluid (liquid/gas) to be measured enters the inlet nozzle 11 and passes through SP_^. impactor chamber 21» While the preferred embodiment is constructed to operate with air as the sample fluid, those skilled in the art will recognize that many of the principles of the present invention, including coincidence detection, might be similarly applied to liquids.

Impactor 17a is cooperatively located within impactor chamber 21 so as to cause particles larger than 5 micrometers to collide with the impactor disc 17a and allow particles smaller than 5 micrometers in size to follow the fluid stream (indicated by hatched line 80 on Fig. 2), thereby passing through the impactor system into the optical particle counter chamber 40. Impactor 17a is mounted on impactor mounting member 17b which has a series of holes formed therein about its periphery through which the fluid flow occurs. Mounting member 17b also includes an indentation in which impactor 17a is inserted.

Impactor 17a thereby establishes an upper particle detection limit which the device 10 can measure. By establishing this limit, the user of device 10 is provided with additional information. This additional information includes two distinct size ranges. A first size range extending from a lower counting size to an upper counting size and a second range extending from an upper counting size to the impact limiting size.

Impactor 17a also serves a second function. The second function is to block room light, or other ambient/stray light, from entering into the apparatus 10 via inlet nozzle 11 and saturating, or causing a signal to be produced by, the photoelectric devices 34, 35 (described below) .

The theoretical limit in size for particle detection by apparatus 10 is approximately 0.05 micrometers in diameter (assuming an air sample at one atmospheric pressure). See, Knollenberg and Luehr, Open Cavity Laser "Active" Scattering Particle Spectrometry, Fine Particles, Ed. by Liu, Academic Press, Inc., New York, (1976) pp. 669-696. However, the practical limit of this technique is usually acknowledged to be 0.1 micrometers. Impactor devices are well known to those skilled in the art and so will not be described further herein. The fluid flow path to impactor chamber 21 is comprised by cap 12. Cap 12 is comprised of inlet port 11, taper section 18 and inlet section 19. Cap 12 is threadedly inserted onto mounting section 20 providing for the cap's 12 removal. By removing cap 12, impactor 17a may be removed/cleaned to eliminate any particulate contamination occurring over a period of time.

Proceeding from the impactor chamber 21, the sample fluid enters into a detector/laser chamber 40 via inlet tube 22. Inlet tube 22 is preferably in fluid communication with impactor chamber 21 and detection chamber 40, and is constructed of pulled and formed tubing. The design considerations of the tubing including the capabilities to withstand the fluid flow and pressures without collapsing or kinking. The detection/laser chamber 40 is comprised of two spherical mirrors 41, 42 (best seen in Figs. 2, 3 and 5).

The spherical mirrors 41, 42 are used to reflect the light scattered from the particles contained in the sample aerosol onto the detectors 34, 35. Spherical mirrors 41, 42 have the ability to refocus a larger sample area onto the detectors 34, 35 than other types of devices commonly used by prior art devices. As those skilled in the art will recognize, preferably mirrors 41, 42 are used which provide for high reflectivity at the wavelength of the light used in the apparatus 10. Also, the type of coatings used on the mirrors 41, 42 must be considered for their resistance to scratching, and the angle of incident light to be reflected. Spherical mirrors 41, 42 enchamber greater than 90% of the scattered light from particles thus allowing device 10 to be very efficient in light collection. In the preferred embodiment spherical mirrors 41, 42 are utilized which are arranged and configured with a 3.366 inch diameter from mirror to mirror. The mirrors 41, 42 are of the vacuum deposited gold plating on a nickel substrate type. Those skilled in the art will recognize that although spherical mirrors 41, 42 are used to refocus the light at detectors while encompassing the sensing volume, other mirrors, such as ellipsoidal mirrors, may be used as well as other reflector devices or optics, holographic optical elements, etc. By providing for a larger sample volume and the imperfect focusing characteristics of spherical mirrors (as compared with that of elliptical or parabolic mirrors) the fluid flow through apparatus 10 may be increased and thus slowed in velocity. In turn, a larger signal is generated by individual particles moving through the sensing volume than would be generated by the same particle in a similar flow volume, high velocity system.

Holding spherical mirrors 41, 42 in place are corner members 61 which rest against vacuum sealing plates 62 and mirrors 41, 42. Soft O-rings 28 are located in channels formed in members 61 and against vacuum sealing plates 62 to apply pressure to spherical mirrors 41, 42 to hold mirrors 41, 42 in proper position. Those skilled in the art will recognize that mirrors 41, 42 are preferably fixedly mounted within a vacuum tight environment. In the preferred embodiment, the environment is formed by a box¬ like container comprised of top 65, bottom 36, and sides 62. The box-like container has holes formed therein for introduction and exit of the sample fluid and light. As best seen in Figs. 5a and 5b, spherical mirrors 41, 42 are approximately semi-spherical in shape, with each mirror 41, 42 having a series of holes or portion of holes formed therethrough. The mirrors being constructed identically to one another. The first hole 70 is located to provide for a detector device 34, 35 (described below to extend into the detection chamber 40. The four holes 71, 72 about the periphery provide for the introduction of the laser beam into chamber 40 (one hole as an entrance and one hole as an exit) with the additional holes being provided for inlet nozzle 22 and exit nozzle 27.

Referring still to Fig. 2, as the sample fluid enters the chamber 40, it is accelerated through tip 23 from inlet tube 22. The sample then passes through a focused beam of light which provides a high power density of light, the intersection of the fluid and light comprising a sensing volume. Preferably the light is an elliptical laser beam and the sensing volume is located between two photo detectors 34, 35. The use of two detectors 34, 35 doubles the amount of light collected and does not appreciably raise the noise level. The noise level immunity is raised by a factor of the square root of two (2). This apparent lowered noise level allows the device 10 to have a higher signal/noise ratio than that of a single detector system.

The use of a plurality of detectors also allows the particle counter 10 to have a greatly reduced zero count when no particles are present in the fluid. Those skilled in the art will realize that photoelectric devices are susceptible to ionizing radiation (e.g., alpha, beta, gamma radiation as well as high energy particles). In a single detector system, ionizing radiation striking the photo detector can cause outputs that are indistinguishable from particle signals. Thus, a single detector device may indicate the presence of particles in a sample aerosol, when in fact there are none.

A two detector system allows counting only signals detected by both detectors. This coincidence technology makes possible an effectively zero background count rate by qualifying the event, wherein the event is the generation of a signal by the detector means. By qualifying the event, apparatus 10 only counts when a signal is detected by both sensing devices 34, 35 within a certain time period. Due to the very low probability that both detectors 34, 35 will be struck by high energy ions at the same time, the system 10 only counts events when the light scattered from the particles being sampled strikes both of the detectors 34, 35. Further, each detector 34, 35 may be established closer to its noise limits. Preferably, a period of approximately 60 microseconds is used. However, a shorter time span may be utilized in order to reject more spurious signals. Those skilled in the art will recognize that a shorter time signal may begin to reject signals generated by particles passing through the sensing volume. Therefore, such time is dependent upon the electronics of apparatus 10 and the size of the particles.

The detector means utilized generates an electrical pulse from incident light. Preferably, such detector means are comprised of a PIN silicone photo-diode of the type manufactured by Hamamatsu, designated by model number S1723 series. The preferred detector is a large area detector. Those skilled in the art will recognize that photo- multiplier tubes or small area detectors might similarly be used among others.

As illustrated in Fig. 6, light scattered by a particle upon entering the sensing volume is reflected by mirrors 41, 42 and is focussed onto detectors 34, 35. The mirrors 41, 42, due to their encompassing of the sensing volume, enclose practically 4 π steradians about the sensing volume. As noted above, mirrors 41, 42 should be designed and positioned so as to deliver, as nearly as possible, all of the components of light scattered by a particle passing through the sensing volume. Here, by way of example only, a light ray is illustrated by hatched line 81a starting from sensing volume, reflecting off mirror 42 and onto detector 35. The symmetrical ray 81b reflects off mirror 41 onto detector 34. The detectors 34, 35 then generate a signal to be qualified by coincidence detection means .

Next, after passing through the sensing volume, the sample fluid exits from chamber 40 via exit tube 26 and exhaust port 27. As those skilled in the art will realize, the flow volume of the sample fluid through device 10 is maintained at a steady rate by a vacuum pump (not shown) and critical orifice (not shown) or some other vacuum creating means. Additionally, sampling pressurized lines or a high pressure source at the input might similarly be used to move the sample fluid through device 10.

In order to limit the maximum counting efficiency of the device 10 to 100%, some means of insuring the sample fluid only passes through the viewing volume once must be undertaken. Commonly, a sheath air system is used to encompass and direct the fluid flow through the sample volume and out of the system. However, device 10 uses a uniquely designed exit nozzle 27. Exit nozzle 27 provides flow dynamics which greatly limits the fluid recirculation by holes 75 formed through the exit nozzle 27. Through various placements of the holes 75 and exit nozzle 27, reduction in the fluid recirculation may be achieved.

Therefore, the mirrors 41, 42 and detectors 34, 35 together comprise sensing means which collect and detect the light scattered by a particle passing through the sensing volume. A signal/event is generated which is qualified by^" coincidence determining means 100 (described below) .

Turning next to Figs. 3 and 6, since low stray light is an important consideration in a device capable of measuring low particle concentrations and sizes, proper care must be taken in formation of the illuminating laser beam. The present invention includes fringing light reduction means which comprise an aperture 41 design specific to the laser 42 and optical system 43. The aperture 41 was modeled and tested to ensure that beam 50 (best seen in Fig. 6) enters the chamber 40 with minimal fringing (i.e., light power outside the desired beam volume). The beam 50 must have finite boundaries because of the spherical mirrors 41, 42. If the beam 50 were to strike the highly reflective spherical mirrors 41, 42, that light would "reflect" around the chamber 40, eventually reaching the photoelectric detectors 34, 35 and saturate them or slightly bias them.

The aperture 41 design was determined by starting a ray tracing system from the view of the field of view of detectors 34, 35. From such ray tracing, and subsequently blocking all primary and secondary reflections, the most efficient and compact system can be constructed. In the preferred embodiment, the aperture 41 is comprised of two circular members 76a, 76b with apertures 77a, 77b formed therethrough. The apertures 77 are sized to allow for passage of the beam while blocking the beam fringing. The apertures 77 themselves are also circular in shape. Those skilled in the art will recognize that the size is dependent upon a number of factors including the detectors 34, 35, mirrors 41, 42 and laser utilized. Laser diode 42 is located such that collimating lens and ball-and-socket means 44 (kugelpfanne) guide the light sequentially through the aperture and sensing volume and onto the light stop device 39. The drive circuitry (not shown) for laser 42 may be located on printed circuit board 45. Laser 42, optics 43, guide 44 and aperture 41 are all located in housing 90 which is preferably vacuum tight and prevents stray/ambient light from entering the system.

Still referring to Fig. 3, also important to minimize background light, is the proper termination of the laser beam. The preferred embodiment utilizes for this function a light termination means comprised of surface 39 coated with a dye based paint having an appropriate refractive index and high absorption at the laser 42 wavelength. The coated surface 39 is oriented at the Brewster, or critical angle, determined by the index of refraction of the paint to air. Virtually all of the light is absorbed by surface 39. This provides for a lower cost and reduces the size and weight of the system.

The dye preferably used is highly absorbing at the design wavelength (here 780 nanometers). The dye utilized has been found to be non-pigmented and provides a smooth surface to insure that no additional scattering occurs.

Alternatively, a simple glass based light dump system oriented at its Brewster angle may be used.

Since a Brewster angle works only for a certain polarization of light, and a laser is not perfectly polarized, a highly absorbing substance at the wavelength of the laser is used to absorb the reflected light with housing 37 from the Brewster-angle surface. The standard anodizing dye used to protect the metal of housing 37 from corrosion is highly reflecting for this wavelength. However, if one alters the process and adds a dye to change the outcome color of the anodizing process, then one can selectively design a broad band wavelength absorbing coating. Therefore, preferably a finish which is highly absorbing at the laser 42 wavelength, in this case a dye utilized in the anodizing the aluminum housing 37, is used.

An additional painting step might also be performed but may add additional cost and steps.

The reason that a low stray light, or background light, on the detectors 34, 35 is important is because the noise level of the system is dependent upon the noise level of the detectors 34, 35. The noise level of the detectors 34, 35 is set by the shot noise in the detectors 34, 35. This shot noise is from two things: (1) the fundamental dark current or leakage current the detector experiences and (2) the extra current the detectors carry because of a constant light level they experience. If one can keep the noise limited to the dark current in the detectors the system can operate with a higher signal to noise ratio.

Turning next to Fig. 4, there is illustrated a preferred embodiment of a functional block which practices the coincidence circuitry of the present invention. The first detector 34 is connected to gain and filter 101a. Gain and filter 101a is in turn connected to gain and filter 102a and to summing device 110. The second detector 35 is connector to gain and filter 101b, which is in turn connected to gain and filter 102b and summing device 110. The outputs of gain and filter devices 102a, 102b are connected to comparators 104a and 104b respectively, while also connected to summing device 103. The output of summing device 103 is provided to comparator 105. Also providing an input to comparator 104a is epsilon 1 (El), while providing input to comparator 104b is epsilon 2 (E2). Providing input to comparator 105 is small particle sum ("SPS"). The output of comparators 104 and 105 are provided to logic block 106. Logic block 106 provides a small particle trigger pulse ("SPT") to block 107. Comparators 104a, 104b also provide output to summing device 108, the output of which enters negative edge enable block 109. The output of negative edge enable block is connected to output control block 107.

Gain and filters 101a, 101b also provide a signal to summer device 110 as noted above. Summing device 110 provides a signal to comparator 111. Large particle sum ("LPS") also provides input to comparator 111. The output signal of comparator 111 is utilized as a large particle trigger ("LPT") pulse to block 107. In operation, when a positive pulse simultaneously strikes detectors 34 and 35, the detectors 34, 35 each provide an output pulse responsive to the amount of light incident, or striking, the detector 34, 35. The output pulse from the detectors 34, 35 are provided to gain and filter blocks 101a and 101b respectively. In the event that the pulse is large (i.e., a large particle on the order of magnitude of 0.50 mm) then summing device 110 provides a large enough signal to activate comparator 111. The signal is also passed through gain and filters 102a, 102b, summed at summing device 103 and provided to comparators 104a, 104b. If the signal output from summing device 103 activates comparator 105, with respect to SPS, then logic block 106, by ensuring that all three outputs of comparators 104a, 104b and 105 are each high (i.e., signals present), provides a signal SPT to output control block 107. If either of the inputs 1 or 2 of logic block 10b are not high within a certain time period, then the event is qualified as a non-particle event.

Those skilled in the art will recognize that either positive or negative logic might be used and that various signal comparators might be used. Further, the magnitude of SPS and LPS might also be adjusted. The coincidence determining means are designed in large part to qualify events so as to eliminate false signals. Such false signals typically occur at a low rate with varying amplitude. Those skilled in the art will also realize that the triggering of an LPT signal causes a qualification of the signal by logic block 106, while a large particle count is eventually counted (if the event is qualified as an actual particle) at block 107. Negative edge enable block 109 may also provide a reset function for block 107 at the end of an event.

Summing device 108 is also provided with the signals from comparators 104a, 104b which provides a negative edge enable to block 107 in order to provide a voltage output signal of different time bases for signals SPT or LPT. The output signal, SPT or LPT, for a qualified event is provided to a processor (not shown) which preferably correlates the time versus concentration of detected particles for user reference. Such mapping may also preferably occur in two different particle size ranges as described above with one graph being provided for each particle size.

In order to further reduce electrical noise and performance of the apparatus 10, the electrical components may be shielded by cans 32, 33 mounted on the printed circuit board 30, 31. Those skilled in the art will recognize that cans 32, 33 are preferably a conductive enclosure, but that other shielding techniques may be used. While not specifically detailed in Figs. 4 and 5, it will be understood that all the functional blocks, laser drive and other electrical components are properly connected to appropriate bias and reference supplies so as to operate in their intended manner. Further, the laser drive of device 10 preferably provides constant power output to the laser.

It is to be understood that even though numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with details of the structured function of the invention, the disclosure is illustrative only and changes may be made in detail, especially in matters of the arrangement of the flow path through the device and the supporting hardware and software routines including the use of positive or negative logic and comparisons, to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.

Claims

WE CLAIM :

1. A continuous flow optical particle counter apparatus of the type wherein a sample fluid is passed through a beam of light, the intersection of the fluid and

5_. beam of light forming a sensing volume, and wherein particles suspended in the sample fluid scatter light when passed through the beam of light, the particle counter comprising:

(a) a plurality of sensor means for collecting 0 scattered light and generating signals responsive to the scattered light; and

(b) coincidence determining means, cooperatively connected to said sensor means, for comparing said generated signals to one another so as to count only 5 those signals which coincide, whereby only events which trigger a plurality of said sensor means are counted.

2. The optical particle counter of claim 1, wherein said sensor means comprise: 0 (a) detecting means operatively connected to said coincidence determining means, for generating a signal responsive to scattered light incident on said detecting means and transmitting said signal to said coincidence determining means; and 5 (b) collecting means for delivering the scattered light from the sensing volume to said detecting means.

3. The optical particle counter of claim 2, wherein said collecting means comprises a plurality of mirrors 0 arranged and configured so as to reflect the scattered light onto said detecting means.

4. The optical particle counter of claim 2, wherein said collecting means comprises transmitting optics arranged and configured to direct the scattered light onto 5 said detector means.

5. The optical particle counter of claim 3, wherein said plurality of mirrors comprises spherical mirrors which refocus the scattered light onto said detecting means.

6. The optical particle counter of claim 5, wherein said spherical mirrors enclose the majority of the sensing volume.

7. The optical particle counter of claim 1, wherein said sensing volume is located within a detection chamber having an inlet and an exit, further comprising impactor means, cooperatively located at said inlet of said detection chamber, for blocking ambient light and establishing an upper particle size limit measured by the counter for particles entering the sensing volume.

8. The optical particle counter of claim 1, wherein the beam of light passes through an aperture formed through a member, said member located prior to the sensing volume, and arranged and configured to block fringing light from the beam of light, whereby fringing light tends to be eliminated and only scattered light is directed onto said detector means.

9. The optical particle counter of claim 8, wherein said aperture is comprised of a plurality of apertures aligned in a side by side axial arrangement.

10. The optical particle counter of claim 1, further comprising a light absorbing device, said device including material aligned at the Brewster angle of said material with respect to the beam of light.

11. The optical particle counter of claim 10, wherein said material is coated with a light absorbing material used at its Brewster angle, whereby said material acts as a highly absorbing coating for the light.

12. The optical particle counter of claim 11, wherein said sensing volume is located within a detection chamber having an inlet and an exit nozzle and wherein said exit nozzle includes holes formed therethrough which tend to limit recirculation of the fluid within the detection chamber, whereby the fluid tends to pass through the sensing volume only once.

13. An optical particle counter apparatus of the type wherein a sample fluid is passed through a beam of light, the intersection of the fluid and beam of light forming a sensing volume, and wherein particles suspended in the sample fluid scatter light when passed through the beam of light, the particle counter comprising:

(a) a detection chamber surrounding the sensing volume, said chamber having an inlet and an outlet for the sample fluid; and

(b) impactor means, cooperatively mounted on said inlet, for blocking particles above a given size and for eliminating the entrance of ambient light into said detection chamber.

14. The optical particle counter of claim 13, wherein the beam of light passes through an aperture formed through a member, said member arranged and configured to block fringing light of the beam of light, whereby only scattered light is directed onto said detector means.

15. The optical particle counter of claim 14, further comprising a light absorbing device, said device including material aligned at the Brewster angle of said material with respect to the beam of light.

16. The optical particle counter of claim 15 wherein said material is coated with a light absorbing material used at its Brewster angle, whereby said material acts as a highly absorbing coating for the light.

17. An optical particle counter apparatus of the type wherein a sample fluid is passed through a beam of light, the intersection of the fluid and beam of light forming a sensing volume, and wherein particles suspended in the sample fluid scatter light when passed through the beam of light, the particle counter comprising:

(a) light stop means, located within the path of the beam of light and subsequent to the sensing volume, for absorbing the beam of light after intersection with the sensing volume, said light stop means utilizing a Brewster angle.

18. The optical particle counter of claim 11, wherein said material is coated with a light absorbing material used at its Brewster angle, whereby said material acts as a highly absorbing coating for the light.

19. The optical particle counter of claim 18, wherein said sensing volume is located with a detection chamber having an inlet and an exit nozzle and wherein said exit nozzle includes holes formed therethrough so as to limit recirculation of the fluid within the detection chamber, whereby the fluid tends to pass through the sensing volume only once.

20. An optical particle counter apparatus of the type wherein a sample fluid is passed through a beam of light, the intersection of the fluid and beam of light forming a sensing volume, and wherein particles suspended in the sample fluid scatter light when passed through the beam of light, the particle counter comprising:

(a) a detection chamber surrounding the sensing volume, said chamber having an inlet and outlet for the sample fluid; and (b) an exit nozzle cooperatively located on said outlet, said exit nozzle having holes formed therein, arranged and configured so as to tend to limit recirculation of the fluid within said detection chamber, whereby particles tend to pass through the sensing volume only once.

21. A method for counting particles, of the type wherein a sample fluid is passed through a beam of light, the intersection of the fluid and beam of light forming a sensing volume, and wherein particles suspended in the sample fluid scattered light when passed through the beam of light, said method comprising the steps of:

(a) collecting the light scattered by said particles;

(b) delivery the light onto a plurality of detectors;

(c) generating signals responsive to said delivered light; and (d) comparing said generated signals to one another so as to qualify and count only those signals which coincide, whereby only events which trigger a plurality of signals are counted.