Abstract:
A condensation nucleus counter (CNC) device implemented with a multi-directional fluid flow system is configured in a compact, conical geometry. A saturation region includes an inlet that delivers into an annular pool a sample stream of air containing small-diameter particles. The stream of air flows out and expands radially from the inlet and mixes with a working vapor to become saturated in the saturation region. The saturated fluid mixture then flows to a condensation region defined by spaced-apart inner and outer radially converging walls having a conical geometry, thereby forming an annular flow volume of decreasing cross sectional area in the direction of fluid flow to the outlet of the CNC device. The working vapor condenses on the small-diameter particles to enlarge their sizes with less tendency to condense on the inner walls of the condensation region.

Description:
TECHNICAL FIELD 
     This invention relates to particle detection and, in particular, to particle detection achieved by a condensation nucleus counter that implements a multi-directional fluid flow system to enlarge condensation nuclei of submicron particles entrained in a fluid stream such that they can be detected and counted. 
     BACKGROUND OF THE INVENTION 
     The current technological trend toward the design of electronic and other devices with increasingly smaller device dimensions has created an increased need for detecting a presence of submicron diameter contaminant particles in facilities where such devices are manufactured. Such need is especially urgent in the field of microelectronic device fabrication, in which the presence of particulate contaminants results in a significant reduction in product yield. 
     Use of a condensation nucleus counter (CNC) represents one method of detecting the presence of submicron particles. In CNC devices, a process of diffusion thermal cooling of a fluid stream carrying submicron particles enlarges the diameters of the particles to sizes that allow their detection using conventional techniques. During thermal diffusion cooling, a submicron particle-carrying fluid sample is passed over a heated pool of volatile liquid, resulting in saturation of the fluid sample with volatile liquid vapor. The resulting vapor-fluid sample mixture is then cooled by thermal diffusion from the cold walls of a condenser. This cooling results in condensation of the vapor onto the surfaces of the submicron particles, thereby enlarging them to form droplets of sufficient size to allow optical detection of the particles. 
     The resolution afforded by a CNC device depends upon droplet size. Droplet size is a function of condensation time, which is dependent on the flow rate of the vapor-fluid sample mixture and the length of the flow path through the condenser. Specifically, reducing the flow rate causes maximum supersaturation to occur at a location closer to an inlet tube of the CNC device. A shorter flow path facilitates the design of a lightweight, lower cost instrument. However, reduction of the flow rate also results in increased processing time, which leads to inefficient and more costly particle detection. 
     Thus, a first problem encountered in prior art CNC devices is an inability to reconcile these competing benefits and detriments to arrive at a lightweight, low-cost instrument that performs efficient and cost-effective particle detection. A second problem with prior art CNC devices is that the submicron particles have a tendency to bounce off the impaction stage and become re-entrained in the flow system of CNC devices. The number of incidences of so-called “particle bounce” events increases with flow rate. One prior art method of minimizing particle bounce is described in U.S. Pat. No. 5,659,388 and entails applying grease to the impaction stage. This method is of limited utility because it introduces contaminants into the CNC device and thereby renders it unsuitable for use in many high-technology industries including microelectronic device fabrication. 
     What is needed, therefore, is a lightweight, low-cost CNC device implementing an improved method for efficiently and cost-effectively detecting the presence of submicron particles while limiting the incidence of particle bounce. 
     SUMMARY OF THE INVENTION 
     An object of the invention to provide a compact, lightweight CNC device that uses a multi-directional, and preferably radial, fluid flow system to enlarge condensation nuclei of submicron particles carried by a sample fluid stream for detection. 
     Another object of the invention is to provide such a device that forms a radially expanding fluid flow path in a working fluid saturation region to increase diffusion time of the working fluid into the sample fluid to form a saturated fluid mixture. 
     A further object of the invention is to provide such a device that implements a radially convergent fluid flow in a condensation region to decrease the length of the fluid flow path. 
     Still another object of the invention is to provide such a device that incorporates a conical geometry in the condensation region to form an annular fluid flow path of decreasing cross sectional area to increase the velocity of the sample fluid stream as it flows through the condensation region. 
     Yet another object of the invention is to provide such a device that operates within a range of above ambient fluid pressures to count with high resolution a greater percentage of entrained particles. 
     The present invention is a condensation nucleus counter (CNC) device implemented with a multi-directional, and preferably radial, fluid flow system. A preferred embodiment of the CNC device includes a saturation region and a condensation region configured in a compact, conical geometry. The saturation region includes a working fluid pool and an annular fluid pool separated by a working fluid—working vapor interface. The saturation region is heated to form from the pool of working fluid a vapor that enters the annular fluid pool. A preferred working fluid is a liquid, such as water, which evaporates to form water vapor. The saturation region includes a tubular inlet that exhausts a sample stream of air containing small-diameter particles into the annular fluid pool above a working fluid surface, which defines the working fluid—working vapor interface. The stream of air flows out radially from the inlet, expands across the working fluid surface, and mixes with the working vapor to become saturated in the saturation region. The flow velocity of the air stream slows as it expands and thereby increases the diffusion time of the working vapor to form the saturated fluid mixture. 
     The saturated fluid mixture then flows through an annular inlet to the condensation region and out of the CNC device through a tubular outlet. The condensation region is defined by spaced-apart inner and outer radially converging walls having a conical geometry, thereby forming an annular flow volume of decreasing cross sectional area in the direction of fluid flow from the inlet to the outlet. The working vapor condenses on the small-diameter particles to enlarge their optical scattering cross sections with less tendency to condense on the inner and outer walls of the condensation region. Thus, particles having diameters on the order of nanometers are the seeds for droplets having diameters on the order of microns. 
     The measurement resolution of the CNC device depends of the sizes of the particles to be detected. Particle size is a function of the fluid flow rate and the length of the flow path through the CNC device. The radial fluid flow system and conical geometry of the condensation region ensure that the residence time of the particles within the CNC device is great enough to achieve supersaturation of the particles in a compact device. An alternative embodiment receiving and processing a sample stream at above ambient pressures counts a greater percentage of particles because the higher fluid pressure contributes to a minimization of working vapor dropout onto the interior flow surfaces of the condensation region. 
     Additional objects and advantages of this invention will be apparent from the following detailed description of preferred embodiments thereof which proceeds with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1,  2 , and  3  are respective side elevation, top plan, and sectional views of the CNC device of the present invention. 
     FIG. 4 is a sectional view taken along lines  4 — 4  of FIG.  3 . 
     FIG. 5 is a sectional view taken along lines  5 — 5  of FIG.  3 . 
     FIG. 6 is a sectional view taken along lines  6 — 6  of FIG.  3 . 
     FIG. 7 is a diagram showing a fluid control system that provides a working fluid to a working fluid pool of the CNC device of FIGS. 1-3. 
     FIG. 8 is an exploded view of certain mounting fixtures and pneumatic and hydraulic components of the CNC device of FIGS.  1 - 3 . 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     A preferred embodiment of the invention is a condensation nucleus counter (CNC) device that is commonly used to monitor and control the quality of air contained within certain environmentally controlled areas such as, for example, a clean room of a semiconductor manufacturing facility. A CNC device detects airborne particles that could cause contamination and lower the product yield of such manufacturing facilities. The airborne particles are enlarged within the CNC device by a process in which nucleation and condensation cause the airborne particles to become enlarged enough to be detected. Nucleation occurs when a sample stream of air in which the airborne particles are entrained becomes saturated with a working fluid vapor. Condensation onto the airborne particles occurs when the saturated sample stream of air reaches higher saturation or supersaturated conditions. The enlarged particles can then be detected and counted by the particle counter. 
     FIGS. 1,  2 , and  3  show a CNC device  10  that includes a containment vessel body  12  composed of a top or condenser portion  14  and a bottom or saturator portion  16  separated by a thermally insulating gasket material  18 . Top and bottom portions  14  and  16  are preferably made of 6061-T6 black anodized aluminum because of its thermal conductivity properties, and gasket material  18  is preferably made of Teflon® material because of its corrosion resistance properties. A sample fluid stream such as, for example, a sample stream of ambient air in which small-diameter particles are entrained flows to a sensor portion  22  of an optical particle counter that detects and counts a number of particles representing the concentration of small-diameter airborne particles entrained in the fluid stream. The optical particle counter is preferably of a 90° scattering detector type that is implemented without a light reflecting element. Body  12  includes a saturation region  24  and a condensation region  26  through which the sample air stream flows so that the particles entrained in the air stream become enlarged and can be more easily detected and counted by the optical particle counter. Body  12  houses a flow distributor  28  of generally conical shape. Flow distributer  28  has an outer surface upper wall  30  that together with an inner wall section  32  of top portion  14  of body  12  forms an annular fluid flow channel  34  for condensation region  26  and has an outer surface lower wall  36  that together with a top surface  38  of a working fluid pool  40  contained within lower portion  16  of body  12  forms an annular fluid pool  42  for saturation region  24 . 
     Annular fluid pool  42  receives at its center an insulated inlet tube  44  through which the sample air stream flows at a sample air stream velocity. The sample air stream flows downwardly out of inlet tube  44  and radially throughout annular fluid pool  42 , as indicated by flow direction arrows  50  in FIGS. 3 and 4. Working fluid pool  40  is held by a lower inner wall section  46  of bottom portion  16  of body  12  and contains a working liquid such as, for example, water or alcohol. Saturation region  24  is thermally conductive and preferably is heated so that the working liquid contained in working fluid pool  40  evaporates. An electric heater  52  formed in the shape of an annulus is bonded to a flat bottom surface  54  of bottom portion  16  of body  12 . Heater  52  is of an electrical resistance type formed with silicone rubber-covered heater wires to which a DC voltage is applied. One such heater is a Model No. 047047 U1 47/99 manufactured by Heatron. The operating temperature of saturation region  24  is preferably about 45° C. 
     As described above, insulated inlet tube  44  that is centrally located in body  12  delivers the stream of sample air for radial outward expansion through annular fluid pool  42  of saturation region  24 . As the working liquid contained in working fluid pool  40  evaporates, the resulting vapor saturates the stream of sample air flowing along radial fluid flow path  50  to form a saturated fluid mixture. Nucleation occurs at this point in saturation region  24  as the airborne particles in the fluid mixture become saturated with the working liquid vapor. 
     The saturated fluid mixture then flows through an annular entrance  56  to condensation region  26 . Inner wall section  32  of top portion  14  of body  12  and outer surface upper wall  30  of flow distributor  28  are conically shaped and are spaced apart to define annular flow channel  34  for the saturated fluid mixture. The spacing between inner wall section  32  of body  12  and outer surface upper wall  30  of the flow distributor  28  remains substantially constant along the length of annular flow channel  34 . The conical shapes of inner wall section  32  and outer surface upper wall  30  define annular flow channel  34  of decreasing cross sectional area in the direction of fluid flow through condensation region  26 , as best seen in FIGS. 4-6. The direction of fluid flow is indicated by direction arrows  60  in FIG.  1 . Condensation region  26  is cooled by four thermoelectric coolers  64  mounted on top portion  14  of body  12  to cause the working fluid vapor in the saturated fluid mixture to become supersaturated and condense on the airborne particles so that they expand in optical scattering cross section (i.e., size) before exiting body  12  through a tubular exit port  62  to sensor portion  22  of the optical particle counter. Thermoelectric coolers  64  are preferably spaced equidistantly around and mounted to a flat top surface  66  of top portion  14  of body  12 . One such type of thermoelectric cooler  64  is a Model No. ST171-12L-1 manufactured by Mekor. In operation, each thermoelectric cooler  64  develops a heat gradient across its height dimension such that a surface  68  contacting flat top surface  16  is cold and an opposite surface  70  is hot. Surface  70  is in thermal contact with a heat sink  72  (FIGS. 2 and 8) to dissipate the heat produced. The operating temperature of condensation region  26  is preferably about 9° C. Saturation region  24  and condensation region  26  are thermally insulated from each other by insulating gasket  18  extending between confronting peripheral surfaces of top and bottom portions  14  and  16  of body  12 . Flow distributor  28  is also insulated and serves to thermally separate saturation region  24  and condensation region  28 . 
     The above-described embodiment of CNC device  10  is configured in a conical geometry with a radially expanding sample fluid stream inlet flow across a heated liquid bath in saturation region  24  followed by a radially convergent flow through a cooled annular condensation region  26 . The radial expansion of the sample fluid stream reduces its velocity before the saturated fluid mixture reaches the transition to condensation region  26 . Decelerating the sample fluid stream flow to a sufficient amount minimizes the dependence of the extent of working fluid saturation on the fluid flow rate. Saturation region  24  and condensation region  26  share a common volume defined by containment vessel body  12  with thermal isolation. Top portion  14  of CNC device  10  is cooled for condensation, and bottom portion  16  of CNC device  10  is heated for saturation. Temperature measuring thermistors (not shown) are positioned at various locations on the outer surface of body  12  at the hot and cold regions (and at inlet tube  44 ) to help regulate their temperatures. The top surface  38  of working fluid pool  40  is constant and, therefore, defines the fluid flow cross section of saturation region  24 . 
     The radial flow system including the radial fluid flow from inlet tube  44  and the radially convergent flow in condensation region  26  provides increased residence time of the fluid mixture within a compact unit. Thus, a lengthy fluid flow path, which would normally be required to increase residence time, is avoided. The radial flow system also reduces the particle velocity within saturation region  24 , allowing more residence time in saturation region  24  to ensure that the particles are adequately saturated by the vapor. 
     Skilled persons will appreciate that a vapor permeable membrane may be positioned at or near top surface  38  of the working fluid to provide an interface between working fluid pool  40  and annular fluid pool  42 . Such a membrane could be made of, for example, Naphion material for use with an alcohol working fluid. No membrane is, however, preferably used with water as the working fluid. 
     An alternative use of CNC device  10  is to operate it with a sample fluid stream flowing at an elevated pressure relative to ambient external pressure. Prior art devices typically use a diffuser to reduce to ambient pressure levels the pressure of a sample fluid stream before its delivery to the inlet tube. Operating CNC device  10  at elevated pressure helps minimize dropout of water vapor onto the cold surfaces in condensation region  26  because of the reduced mobility of the water vapor and air molecules. Reducing such dropout of water vapor results in a greater percentage of detected flow particles and better uniformity of water droplet diameters, the latter providing better particle counting resolution. 
     The use of an elevated pressure condensation region  28  promotes condensation of the working fluid vapor onto the saturated airborne particles and allows small molecular working fluids such as, for example, water to be used without appreciable vapor depletion regions. Depletion of vapor in regions near inner wall section  32  of condensation region  28  makes difficult a 100% supersaturation of the fluid mixture. The use of an elevated pressure in condensation region  28  provides approximately 100% supersaturation of the fluid mixture using small molecular working fluids. 
     Although there is a measurable, insignificant pressure drop between saturation region  24  and condensation region  26 , there is preferably no differential pressure purposefully introduced between them. A preferred range of sample fluid stream pressure is between about 35 psi and 135 psi, as compared with 14.7 psi, representing a typical ambient pressure, depending on the elevation. 
     The final diameter and nucleation threshold of the particles are depend various factors such as, for example, the operating temperatures of the saturation and condensation regions and the operating pressure in the condensation region. Saturation temperatures typically range between about 40° C. and 60° C., and condensation temperatures typically range between about 5° C. and 20° C. The final particle droplet diameters typically range between about 1 μm and 2 μm to give minimum particle detector sensitivities of between about 0.01 μm and 0.02 μm. 
     FIG. 7 is a diagram of a working fluid fill system  80  that provides a constant working fluid level in working fluid pool  40 . The particular example depicted by FIG. 7 shows an operational configuration of working fluid fill system  80  in which CNC device  10  receives a pressurized sample gas stream. With reference to FIG. 7, a reservoir  82  holding a quantity of water, which is a preferred working fluid, has a solenoid-controlled outlet port  84  that supplies the working fluid to one port of a three-port flow valve  86  equipped to which an optical level sensor  88  is connected. The other two ports of flow valve  86  form part of a working fluid circulation flow path of working fluid that a circulating pump  90  propels through working fluid pool  40  by entry into and discharge from, respectively, an inlet port  92  and an outlet port  94 . 
     A sample gas stream delivered at 100 psi through inlet tube  44  into annular fluid pool  42  exerts a downward pressure against the working fluid contained in working fluid pool  40 . A pressure transducer  98  positioned to receive working fluid flowing out of flow valve  86  and into circulating pump  90  monitors the operating pressure inside saturation region  24  and condensation region  26 . The pressurized sample gas stream is also delivered to an inlet  96  at the top of reservoir  82  to counterbalance such downward pressure exerted in working fluid pool  40 . Optical level sensor  88  senses a presence of air in the working fluid flowing through flow valve  86  and thereby produces an output signal indicative of the working fluid level in working fluid pool  40 . A solenoid controller  100  responds to an output signal from level sensor  88  that indicates the presence of air by opening outlet port  84  to start flow of working fluid through outlet port  84  of reservoir  82  in an amount that maintains an operational working fluid level in working fluid pool  40  as the working fluid circulates through it. 
     FIG. 8 is an exploded view of certain mounting fixtures and pneumatic and hydraulic components of CNC device  10 . With reference to FIG. 8, CNC device  10 , together with circulating pump  90  and pressure transducer  98 , is contained by a metal housing box  104  enclosed by side cover plates  106  with apertures through which pneumatic and hydraulic fluid lines can pass. Housing box  104  includes a depression having an upper surface  108  to which heat sink  72  is mounted and a lower surface  110  against which upper surfaces  70  of thermoelectric coolers  64  are positioned for thermal conduction to heat sink  72 . Sensor portion  22  of the optical particle counter projects through an opening  112  in heat sink  72  to exhaust the sample fluid stream through exit port  62 . 
     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.