Abstract:
Fluid-based particle detection exhibits improved light collection and image quality from a light collection system that uses immersed optics on a flow-through cell for collecting and detecting scattered light from particles carried by the fluid. The flow-through cell includes first and second body sections that are coupled to form a unitary article and have opposed interior surface portions configured to form opposed walls of a flow channel through which the fluid flows. First and second optical elements are associated with the respective first and second body sections. In certain embodiments, at least one of the first and second optical elements is an integral part of its associated body section. A lens element constructed as an integral part of the unitary flow-through cell eliminates additional interfaces or bonding joints that cause scattering and absorption of light.

Description:
RELATED APPLICATIONS 
   This application is a continuation-in-part of U.S. patent application Ser. No. 10/927,671, filed Aug. 27, 2004, for PARTICLE DETECTION SYSTEM, abandoned, which is a continuation of U.S. patent application Ser. No. 10/407,650, filed Apr. 4, 2003, now U.S. Pat. No. 6,784,990, for PARTICLE DETECTION SYSTEM IMPLEMENTED WITH A MIRRORED OPTICAL SYSTEM. 

   TECHNICAL FIELD  
   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 INFORMATION 
   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 U.S. Food and Drug Administration requires particle monitoring because of the correlation between detected particles in an aseptic environment and viable and non-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, thereby reducing the size of particles that can cause quality problems. Accordingly, it is important to detect and accurately measure submicron-diameter particles of ever-decreasing sizes and numbers for each volumetric unit. 
   To perform particle monitoring, currently available commercial submicron-diameter particle detection systems use optical detection techniques to determine the presence, size, and number of particles in a volumetric unit. This technology is based on 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 fluid stream containing sample particles. Light scattered by a particle in the view volume is collected with optics and focused onto a detection system that includes one or more detector elements. The detection system includes a light detector that detects the incidence of light and generates a pulse output signal, the magnitude of which depends on the intensity of the scattered light. The magnitude of the pulse output signal is compared to a predetermined pulse output signal 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 that measures the voltage of the pulse output signal and determines particle size therefrom. Consequently, 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-diameter sample particles. 
   What is needed, therefore, is a particle detection system having high sensitivity in detecting submicron-diameter particles. 
   SUMMARY OF THE INVENTION 
   Preferred embodiments of the invention improve light collection and image quality from a collection system of a fluid particle detection system by using immersed optics on a flow-through cell for collecting and detecting scattered light from particles. The particle detection system is capable of optically detecting particles in a fluid stream and includes a flow chamber within which a light beam propagating along a light propagation path and a fluid stream containing sample particles transversely intersect to form a view volume. The incidence of a sample particle with the light beam causes portions of the light beam to scatter from the view volume in the form of scattered light components. At least one scattered light component exits the view volume, is collected and focused by a light collection lens system, and is incident on a photodetector. The photodetector detects the incidence of the scattered light component and generates a pulse output signal correlating to a predetermined parameter (e.g., size) of the scattered light component. 
   The view volume of the particle detection system is located within a flow-through cell that includes first and second body sections that are coupled to form a unitary article. The first and second body sections have opposed interior surface portions that are configured to form opposed walls of a flow channel through which the fluid stream flows. First and second optical elements are associated with the respective first and second body sections. In certain embodiments, at least one of the first and second optical elements is an integral part of its associated body section. A lens element constructed as an integral part of the unitary flow-through cell, when compared to a lens attached to a flat cell wall, eliminates additional interfaces or bonding joints that cause scattering and absorption of light. The unitary flow-through cell is sized for insertion into the particle detection system in an orientation that positions the first and second optical elements along a light collection lens system axis. 
   In certain embodiments of the particle detection system, the flow-through cell includes a pair of spacers positioned between and coupled to the first and second body sections. The spacers are spaced apart from each other to define opposed interior surface portions that form the opposed walls of the flow channel. The spacers are formed of optically transparent material such that they function as ingress and egress windows of the view volume for the light beam. The first and second body sections and the pair of spacers may be fused into an integral structure such that the pair of spacers define surface interfaces between the spacers and the first and second body sections. 
   In certain embodiments of the particle detection system, one of the optical elements is a transparent optical element, lens, or mirror, and the other optical element is a lens. The presence of a lens in the flow-through cell increases the numerical aperture of the light collection lens system. Increasing the numerical aperture increases the collection angle and results in a corresponding increase in amount of light collected by the light collection lens system. An increase in the amount of light collected results in an increase in the magnitude of the pulse output signal generated by the detector. 
   Increasing the magnitude of the pulse output signal for a given particle size allows for detection of a smaller size particle at a given threshold. For a sensor that is not background light noise limited (i.e., the noise is dominated by detector and associated electronic noise), collecting more light will increase the magnitude of the output signal without increasing the noise. The threshold remains the same, but the signals that cross it correspond to smaller particles. Moreover, for a given particle detection size, increasing the magnitude of the signal allows for increasing the threshold farther away from the noise and thereby reduces false counting resulting from randomly occurring noise. Consequently, the pulse output signal threshold for a given false count rate may be increased, and the particle detection system can maintain the desired overall false count rate, since most noise is random and of insufficient magnitude to generate a pulse output signal that has a magnitude greater than the predetermined threshold. 
   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 
       FIG. 1  is a plan view of an exemplary particle detection system implemented with an immersed optical system. 
       FIG. 2  is an enlarged, simplified plan view of an exemplary modified implementation of the particle detection system of  FIG. 1 . 
       FIG. 3A  is an isometric view of an exemplary unitary flow-through cell, and  FIGS. 3B and 3C  are, respectively, simplified plan and side elevation views of the exemplary unitary flow-through cell of  FIG. 3A . 
       FIGS. 4A ,  4 B, and  4 C are, respectively, isometric, plan, and side elevation views of an alternative exemplary unitary flow-through cell. 
       FIG. 5  is an isometric view of the unitary flow-through cell of  FIGS. 3A ,  3 B, and  3 C installed in the particle detection system of  FIG. 1 . 
       FIG. 6  is an enlarged, simplified plan view of an alternative exemplary implementation of the particle detection system of  FIG. 1 . 
       FIG. 7  is a plan view of the particle detection system of  FIG. 6 . 
       FIGS. 8A and 8B  are, respectively, plan and side elevation views showing in cross-section a particle detection system housing in which the unitary flow-through cell of  FIGS. 6 and 7  are installed. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1  is useful in explaining the configuration of an exemplary particle detection system  10 . Particle detection system  10  includes a flow chamber  12  (extending out of the plane of  FIG. 1 ) through which a particle-carrying sample fluid stream  14 , such as gas (e.g., air) or liquid (e.g., water), flows in a flow direction  16  (out of the plane of  FIG. 1 ). Particle detection system  10  also includes a light source  18  emitting a light beam  20  that propagates in a direction along an optical axis  22 . Light source  18  is preferably a diode laser that is robust, efficient, and compact. Exemplary preferred light sources are gas, dye, and solid-state lasers. 
   Fluid stream  14  and light beam  20  intersect within the effective center of flow chamber  12  in a region called a view volume  26 . View volume  26  is located within a unitary flow-through cell  28  that includes first and second spaced-apart, opposed optically transparent windows  30  and  32  through which light beam  20  propagates into and out of view volume  26 . Unitary flow-through cell  28  also includes an optical element  36  that is spaced apart from and positioned in opposition to a lens  38  that collects a scattered light component  40  exiting view volume  26  along a collection system axis  42 . Optical element  36  and lens  38  are positioned on opposite sides of fluid stream  14 . Optically transparent windows  30  and  32  are preferably formed of an optically polished, transparent material, such as fused silica. 
   Optical element  36  is preferably a transparent lens or mirror made of glass, crystal, or plastic material. Scattered light component  40  can pass through optical element  36  or be reflected off of optical element  36  having a curved outer surface  46  coated with a high-reflectance coating material. Lens  38  is positioned between view volume  26  and a light collection lens system  44 . Lens  38  is preferably a transparent, convex lens of elliptical, aspherical, or spherical shape. An outer surface  48  of lens  38  is preferably coated with an anti-reflective coating material. In a preferred embodiment, optical element  36  and lens  38  are of sufficient size that their interfaces with optically transparent windows  30  and  32  are not within the field of view of light collection lens system  44 . 
   Light collection lens system  44  is positioned adjacent to unitary flow-through cell  28  and along collection system axis  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. Light collection lens system  44  collects and images scattered light component  40  onto a light-receiving surface  50  of a photodetector element  52 . Photodetector element  52  is positioned so that its light-receiving surface  50  is substantially perpendicular to collection system axis  42 . Collection system axis  42  is orthogonal to optical axis  22  along which beam  20  propagates. Collecting light for viewing orthogonal to beam  20  facilitates light stop implementation in system  10  to significantly reduce detected stray light from the interfaces of optically transparent windows  30  and  32 . 
   Photodetector element  52  generates a pulse output signal having a magnitude corresponding to the intensity of scattered light component  40 , which is dependent on the size of the particle to which it corresponds. Signal processing takes place downstream of photodetector element  52  and converts the pulse output signal into a voltage that can be measured. Because particle size directly relates to light amplitude, which directly relates to pulse output signal amplitude, which directly relates 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. 
     FIG. 2  is useful in explaining the path of light beam  20  and scattered light components  40   a  and  40   b  as they progress through particle detection system  10  of  FIG. 1 . The width of laser beam  20  is the same as or is smaller than the width of optically transparent window  30 . Laser beam  20  is of the same width when it is desired to illuminate flow chamber  12  in its entirety to achieve particle scattering, and laser beam  20  is focused to a smaller width to increase intensity and thereby enable detection of smaller particles. Typically, the widths of window  30  and beam  20  are the same. Following its incidence on a particle  54  present in view volume  26 , light beam  20  exits view volume  26  as scattered light components  40   a  and  40   b , which initially propagate in generally opposite directions. As shown in  FIG. 2 , a scattered light component  40   a  exits view volume  26  in a direction toward light collection lens system  44 , and a scattered light component  40   b  exits view volume  26  in a direction toward optical element  36 . ( FIG. 2  shows optical element  36  having a flat outer surface  56 , instead of curved outer surface  46  shown in  FIG. 1 .) In the exemplary embodiment shown in  FIG. 2 , scattered light component  40   b  passes through optical element  36  and exits particle detection system  10 . Scattered light component  40   a  exits view volume  26 , is incident on lens  38 , and passes through light collection lens system  44 , which focuses scattered light component  40   a  onto light-receiving surface  50  of photodetector element  52 . 
     FIGS. 3A ,  3 B, and  3 C are respective isometric, plan, and side elevation views of a preferred unitary flow-through cell  28 . Flow-through cell  28  is in the form of two truncated hemispherical solid glass body sections  60  and  62  separated by spaced-apart rectangular spacers  64  and  66 . All four components are assembled preferably by fusion at high temperatures to form fluid-tight seals between adhesive material-free adjacent component interfaces and thereby form a unitary article. If fitted together without spacers  64  and  66 , body sections  60  and  62  would resemble two halves of a sphere that is truncated to have two sets of two opposed planar exterior surface regions of circular shape in which the sets are orthogonally aligned to each other. Body sections  60  and  62  have respective rectangular flat major surfaces  68  and  70 . Flat major surfaces  68  and  70  are bordered by four respective semicircular flat surfaces  72  and  74 , each of which corresponding to one-half of a planar exterior surface region of circular shape. When flow-through cell  28  is assembled, body sections  60  and  62  are spaced-apart by rectangular spacers  64  and  66  positioned between flat major surfaces  72  and  74  to form flow chamber  12  through which sample fluid stream  14  flows. Flow chamber  12  has a rectangular cross-sectional shape defined by opposed interior surface portions  80  of spacers  64  and  66  and opposed interior surface portions  82  and  84  of, respectively, optical element  36  and lens  38 . Optical element  36  and lens  38  are “immersed” in that fluid flows in direct contact against their respective interior surface portions  82  and  84 . Optically transparent windows  30  and  32  are formed by the larger area side surfaces of the respective spacers  64  and  66 . Lens  38  and optical element  36  form portions of the respective body sections  60  and  62 . View volume  26  lies between interior surface portions  80  of optically transparent windows  30  and  32  and between interior surface portions  82  and  84  of, respectively, optical element  36  and lens  38 . The outer surfaces of optically transparent windows  30  and  32  are preferably coated with an anti-reflective coating. 
   Optical element  36  and lens  38  are transparent optical elements that serve to partly confine the liquid flow in sample fluid stream  14  and to confine scattered light component  40  and direct it through light collection lens system  44  such that it is incident on photodetector element  52 . In the preferred embodiment of  FIG. 3 , for lens  38  and optical element  36 , their respective inner surfaces  68  and  70  are flat and their respective outer surfaces  48  and  46  are curved. Curved outer surfaces  46  and  48  are preferably of elliptical, aspherical, or spherical shape.  FIGS. 4A ,  4 B, and  4 C show an alternative preferred embodiment of a chamber  12   a  that has optically transparent windows  30   a    32   b  through which an optical axis  22   a  passes. Chamber  12   a  includes a flow cell  28   a , in which inner surfaces  68   a  and  70   a  and outer surfaces  46   a  and  48   a  are flat. In both preferred embodiments, the outer surfaces of optical elements  36  and  36   a  and lenses  38  and  38   a  interface with air. 
     FIG. 5  is a three-dimensional isometric view of the unitary flow-through cell of  FIGS. 3A ,  3 B, and  3 C installed in the particle detection system of  FIG. 1 .  FIGS. 3A ,  3 B, and  3 C are illustrative of the preferred fluid-tight seals formed between optically transparent windows  30  and  32 , optical element  36 , and lens  38 . The use of unitary flow-through cell  28  in particle detection system  10  minimizes the mechanical interference that causes scattering and absorption of light within particle detection system  10  by, for example, bonding joints. Further, mechanical centering of unitary flow-through cell  28  within the optical system (light collection lens system  44  and photodetector element  52 ) is more precise when using unitary flow-through cell  28  because the square edges of flow-through cell  28  facilitate its placement within particle detection system  10 . The mechanical design features are described below with reference to  FIGS. 8A and 8B . 
   The positioning of a lens along a light collection lens system axis increases the numerical aperture of the light collection lens system, thereby increasing the amount of light collected by the light collection system, the amount of light incident on the light detector element, and, as a consequence, the magnitude of the resulting pulse output signal. When the pulse output signal magnitude corresponding to a detected sample particle is increased, the pulse output threshold level that differentiates noise from valid particle detection signals may be lowered. 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. 
   An exemplary preferred particle detection system that includes a light-reflecting optical element  36 ′ is shown in  FIGS. 6 and 7 . Light-reflecting optical element  36 ′ of flow-through cell  28 ′ is preferably a light reflector in the form of a curved segment having an outer surface that is of spherical, elliptical, or aspherical shape. An outer surface  46 ′ of light-reflecting optical element  36 ′ is coated with a high-reflectance coating. Light-reflecting optical element  36 ′ is preferably a mirror and is positioned opposite light collection lens system  44 , with view volume  26  and lens  38  disposed between them. Light-reflecting optical element  36 ′ is centered on collection system axis  42  such that the center of curvature of light-reflecting optical element  36 ′ is aligned with the effective center of view volume  26 . In a preferred implementation, light-reflecting optical element  36 ′ has a diameter that is the same as the diameter of lens  38 , which arrangement doubles the amount of scattered light collected by light collection lens system  44 . 
     FIG. 6  is useful in the explanation of the processing of scattered light components  40   a  and  40   b  formed by the incidence of light beam  20  on particle  54 . The incidence of light beam  20  on particle  54  scatters correlated light components  40   a  and  40   b  from view volume  26  in respective first and second directions. Scattered light component  40   a  exits view volume  26  in a direction generally toward lens  38  and light collection lens system  44 , and scattered light component  40   b  exits view volume  26  in a direction generally away from lens  38  and toward light-reflecting optical element  36 ′. Scattered light component  40   b  is incident on light-reflecting optical component  36 ′, which acts as a light reflector that reflects and inverts about optical axis  22  scattered light component  40   b . Scattered light component  40   b  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  42  as that of scattered light component  40   a . Both scattered light components  40   a  and  40   b  propagate in a direction along collection system axis  42  through light collection lens system  44  that converges light components  40   a  and  40   b  onto a light-receiving surface  90  of a photodetector array  92 . 
   Photodetector array  92  is positioned, so that its light-receiving surface  90  is substantially perpendicular to, and the number of detector elements in the linear array is bisected by, collection system axis  42 . Collection system axis  42  divides photodetector array  92  into two sets of detector elements, one that contains a first detector element  94  and another that contains a second detector element  96 . Detector elements  94  and  96  are preferably equidistant from collection system axis  42 . 
   Scattered light component  40   a  propagates through light collection lens system  44  and is focused onto detector element  94  of photodetector array  92 . The inverted scattered light component  40   b  propagates through view volume  26  and light collection lens system  44 , which focuses inverted scattered light component  40   b  onto detector element  96  of photodetector array  92 . Detector elements  94  and  96  constitute a related pair of detector elements of photodetector array  92  such that detector element  94  is spatially related to scattered light component  40   a  and such that detector element  96  is spatially related to inverted scattered light component  40   b . Each of detector elements  94  and  96  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. Only those pulse output signals that are temporally and spatially coincident such that both of detector elements  94  and  96  of the pair of detector elements concurrently generate pulse output signals are processed by the signal processing system downstream of the photodetector elements. If each of the pulse output signals concurrently crosses its predetermined 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. If the pulse output signal from either of detector elements  94  and  96  does not exceed the predetermined threshold, the signal is ignored by particle detection system  10 ′ and is not further processed. If the pulse output signals from detector elements  94  and  96  are not coincident, they are ignored by particle detection system  10 ′ and are not further processed. 
   Photodetector array  92  is preferably a linear array of photodiode detectors having dimensions that are proportional to the image dimensions of view volume  26 . 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. An array of photodiode detectors is used for the purpose of detecting coincidence and thereby reducing noise and false counts. 
   While many signal processing systems are known to those skilled in the art, exemplary preferred signal processing systems for use in connection with the particle detection system of the present invention are described in U.S. Pat. No. 6,784,990 to DeFreez et al., which is hereby incorporated by reference. 
   Particle detection systems implemented with an immersed optical system as described above have an increased ability to distinguish between noise and low-amplitude pulse output signals caused by small-diameter particles. Signal enhancement results from the inclusion of lens  38  in unitary flow-through cell  28 . The immersion of lens  38  next to sample fluid stream  14  flowing in unitary flow-through cell  28  increases the numerical aperture of light collection lens system  44  by the index of refraction of the sample fluid medium:
 
 NA=N (sinθ),
 
where NA is the numerical aperture, N is the index of refraction of the medium, and θ is the collection system input half-angle. For example, an exemplary prior art particle detection system constructed with a refractive lens system for collecting light scattered from particles originating from a liquid source, passing through a window, and propagating through air has a numerical aperture of 0.64. When a lens is placed next to the liquid medium (e.g., water), the numerical aperture increases by the index of refraction of the water (1.33). Thus, the numerical aperture of the particle detection system including a lens is
 
 NA =(0.64)(1.33)=0.85.
 
The amount of light collected is increased by 1.9, as computed using solid angles, which is roughly the square of the NA ratio.
 
   Increasing the numerical aperture increases the collection angle and thereby increases the amount of light collected by light collection lens system  44 . An immersed reflector of the same diameter as the diameter of the opposing lens in a system such as that described with reference to  FIGS. 6 and 7  doubles the amount of light collected from a particle. An increase in the amount of light collected results in an increase in the magnitude of the pulse output signal generated by a photodetector element. For the reasons stated above, increasing the pulse output signal magnitude resulting from the detection of each sample particle permits the use of a higher threshold to differentiate noise from valid particle detection signals. The threshold for a given false count rate may, therefore, be increased and the particle detection system can still maintain the desired overall false count rate, since most noise is random and of insufficient magnitude to generate a pulse output signal that has a magnitude greater than the threshold. 
     FIGS. 8A and 8B  are respective plan and side elevation views showing in cross-section particle detection system  10 ′ with flow cell  28 ′ installed in a housing  100 .  FIG. 8A  shows the angular offset of beam axis  22  and collection system axis  42  and exhibits the role of flat major surfaces  68  and  70  in achieving a compact fit within housing  100 .  FIG. 8B  shows the intersection of beam axis  22  with flow chamber  12 , which receives fluid flow from an inlet  102  and discharges fluid flow through an outlet  104 . 
   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 of this invention without departing from the underlying principles thereof. The scope of the present invention should, therefore, be determined only by the following claims.