Abstract:
A method and apparatus for measuring particle concentration and size distribution of particles in liquids. The method involve separating dissolved and particulate residues in liquids for determination of the size and concentration of the particulate species. The method includes the steps of forming an aerosol from the liquid sample to be analyzed, evaporating the droplets in the aerosol to dryness, and detecting the particles. An apparatus for separating dissolved and particulate residues in liquids for determination of the size and concentration of the particulate species is also disclosed. The apparatus includes a droplet former, a dryer communicatively connected to the droplet former, and a detector communicatively connected to the evaporator for detecting particles.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS, IF ANY 
     This application is a divisional of U.S. patent application Ser. No. 12/357,088, filed Jan. 22, 2009, now U.S. Pat. No. 8,272,253, which claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Patent Application Ser. No. 61/011,901, filed Jan. 22, 2008, which is hereby incorporated by reference. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     REFERENCE TO A MICROFICHE APPENDIX, IF ANY 
     Not applicable. 
     BACKGROUND 
     1. Field 
     The present invention relates, generally, to analysis methods and apparatus for use with compositions of matter. More particularly, the invention relates to a method and apparatus for measuring the size and concentration of small particles in high purity liquids and colloidal suspensions. Most particularly, the invention relates to an apparatus and method for separating dissolved and particulate residues in a liquid to determine the size distribution and concentration of the particulate species (i.e. particles). The technology is useful, for example, for accurate measurement of low concentrations of very small (sub 50 nm) particles in high purity liquids, the measurement of particle retention by filters, and measurement of particle size distributions in colloidal suspensions. The invention is suitable for use in the semiconductor device manufacturing industry, the ink manufacturing industry, and in other fields. 
     2. Background Information 
     The present invention has utility in measurement of the concentration of small, for example, sub 50 nm, particles in high purity liquids. Small particles are a major problem for the semiconductor device manufacturing industry. Particles smaller than 50 nm can significantly reduce manufacturing yield of present day semiconductor devices. The ability to measure concentrations, especially low concentrations, of these particles is highly desired. Insofar as is known, there is no technology to meet this need. 
     The present invention also has utility in measurement of particle retention by filters, particularly those with pore sizes smaller than about 50 nm. Microporous membrane filters are often used to reduce particle levels in liquids for semiconductor device manufacturing. The ability of filters of this type to remove particles from the liquids is usually determined by challenging the filters with particles and measuring what comes through. Instruments capable of measuring particles of these sizes are not believed to be available. 
     The invention further has utility in measurement of Particle Size Distributions (PSD) in colloidal suspensions. There are numerous applications in which the size distribution of particles in colloidal suspensions is important in determining the efficacy of the suspension. Examples include slurries used in chemical mechanical planarization (CMP) of silicon wafers, as well as wafers composed of other materials, during semiconductor chip manufacturing and pigment-based inks. The PSD of CMP slurries determines the planarization rate, surface smoothness and scratch density on the wafer surface following the CMP process. All of these are important in determining the finished semiconductor device yield and performance. The size distribution of pigment inks is important in determining color development. 
     Historically, the first application mentioned above, measurement of concentrations of small particles (particularly those less than 50 nm in size) in high purity liquids has been addressed using single particle optical particle counters (OPCs). These instruments size and count individual particles as they pass through a laser beam. They have met the need of the semiconductor industry until recently, although they have typically been believed to have been a half step behind in development. Approximately twenty years ago the industry needed to detect roughly 500 nm particles; now they desire to measure 20-30 nm particles. The problem is that below about 300 nm the amount of light scattered by a particle is proportional to the 6 th  power of the diameter (D P   6 ). Therefore, an instrument to measure 30 nm particles needs to be 1,000,000 times more sensitive than one that measures 300 nm particles. A leading company in making these counters has been Particle Measuring Systems. Their highest sensitivity in water is claimed to be 50 nm, but it is believed to be closer to 60-70 nm. Claimed sensitivity in chemicals is 65 nm. Particle Measurement Systems had a counter with a claimed sensitivity of 30 nm counter on the market at one time, but it is no longer available. Other companies that make counters of this type are RION, Horiba, Particle Sizing Systems, and Hach Ultra. 
     Very small particles (typically smaller than 10 nm) in liquids have also been analyzed using a combination of electrospray and mass spectroscopy. Electrospray is used to generate small droplets by subjecting the liquid to a high electric field. The liquid must be moderately conductive and the droplets become highly charged during formation. High purity liquids typically have low conductivity making the formation of small droplets difficult. Also, the high charge on the particles can result in particle agglomeration and may cause other changes in particle properties. The agglomeration issue can be addressed by exposing the aerosol to ionizing radiation. 
     The second application, measurement of particle retention by filters with small pore sizes, has also been addressed using OPCs, again limited to 50 nm. Other techniques have also been used such as turbidimitry. However, these techniques can only be used at very high particle concentrations, concentrations well above those seen in high purity applications. And, filter performance at these high concentrations is not representative of performance at lower concentrations. 
     Filter performance has also been measured using non-volatile residue monitors (NVRM or NRM). These instruments work by forming an aerosol of the particle-laden liquid, evaporating the liquid in the aerosol and measuring the number of particles in the aerosol. The problem with this method is that any dissolved material in the liquid forms a particle when the liquid is evaporated. Hence, the instrument measures both dissolved and particulate residue. And the residue particles interfere with the particulate particle measurement. 
     The third application, measurement of particle size distributions in colloidal suspensions, has typically been addressed using either dynamic light scattering (DLS) or centrifugal sedimentation. Both of these methods only measure relative PSDs. They cannot determine concentrations. 
     For these and other reasons, a need exists for the present invention. 
     All US patents and patent applications, and all other published documents mentioned anywhere in this application are hereby incorporated by reference in their entirety. 
     BRIEF SUMMARY 
     The present invention provides a method and apparatus for measuring (a) the concentrations of small particles, on the order of 50 nm or smaller, and (b) the size distributions of such particles, in liquids, which method and apparatus are practical, reliable, accurate and efficient, and which are believed to fulfill a need and to constitute an improvement over the background technology. 
     In a basic embodiment, the method of the present invention includes the steps of, providing a specimen to be tested, isolating small, uniformly sized droplets from the specimen, evaporating the droplets to dryness, and counting and sizing the particles that were originally (initially) in the liquid. Thereby, particle concentration and PSD may be determined. The method is especially effective for measuring low concentrations of very small particles, particularly those less than 50 nm in size. 
     In one aspect of the present invention, an apparatus includes a nebulizer/impactor and a condensation particle counter (CPC). The nebulizer/impactor has means to form or isolate small, uniformly sized droplets. The CPC accurately counts particles present after the small, uniformly sized droplets are dried. This embodiment is believed to be best suited for high purity liquid to measure particle concentration above a defined threshold, but without PSD measurement, a threshold particle counter (TPC). 
     In another aspect, the apparatus includes a nebulizer/impactor and a scanning mobility particle sizers (SMPS), which performs both particle counting and sizing. This embodiment is believed to be best suited for determining PSD in addition to particle concentration. 
     In a further aspect, a nebulizer/impactor combination is provided for generating an aerosol composed of multiple droplets of a liquid. The nebulizer/impactor includes a housing forming a mixing chamber having (i) a liquid entrance for receiving a sample liquid into the chamber, (ii) a primary orifice having a first diameter for receiving a pressurized gas into the chamber for merger with the sample liquid to generate an aerosol composed of multiple droplets of the sample liquid suspended in the gas, and (iii) a secondary orifice having a secondary diameter for conducting the aerosol out of the chamber. The second orifice is less than a major dimension of the mixing chamber taken in a direction substantially perpendicular to an axis of the secondary orifice, so as to restrict flow out of the mixing chamber to generate a back pressure in opposition to entry of the sample liquid and the pressurized gas into the chamber. 
     In contrast to other nebulizers in which the chamber exit is simply open to the downstream components with a diameter equal to that of the chamber, the exit orifice in the nebulizer in D has a diameter less than that of the chamber, more preferably less than half the diameter chamber. The diameter reduction provides a constriction that produces a higher kinetic energy mixing of the gas and liquid in the merger zone. As a result, the nebulizer generates smaller droplets. The secondary orifice also helps direct the aerosol toward the impactor surface raising the impactor efficiency. 
     Another factor reducing the droplet size produced by the atomizer/impactor is close axial positioning of an impactor just downstream of the secondary orifice. The more closely spaced impactor removes a greater proportion of the larger droplets. 
     In a preferred version of nebulizer/impactor, the impactor axial spacing from the secondary orifice is adjustable through movement of the impactor. For example, a threaded mounting of the impactor to the nebulizer frame allows axial position adjustment by turning the impactor about its longitudinal axis. The average size of the droplets in the aerosol leaving the nebulizer can be increased or decreased by respectively enlarging or reducing the axial spacing between the secondary orifice and the impactor. The average size can also be decreased and the uniformity increased by making the shape of the housing containing the secondary orifice conformal to the impactor shape. 
     The droplet size produced by atomizer/impactor also can be adjusted by changing or selecting the secondary orifice. Reducing the diameter of the secondary orifice is believed to increase back pressure and reduce droplet size. It has been found useful to provide a secondary orifice with a diameter larger than that of the primary orifice. The ratio of the secondary orifice diameter to the primary orifice diameter can range from slightly above one, to about two in versions that incorporate a secondary orifice. 
     The aspects, features, advantages, benefits and objects of the invention will become clear to those skilled in the art by reference to the following description, claims and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
       The present invention, and the manner and process of making and using it, will be better understood by those skilled in the art by reference to the following drawings. 
         FIG. 1  is a flow diagram of one embodiment of the method of the present invention. 
         FIG. 2A  is a diagram illustrating an embodiment of the apparatus of the present invention. 
         FIG. 2B  is a diagram illustrating an alternative embodiment of the apparatus of the invention. 
         FIG. 2C  illustrates an embodiment of an apparatus for testing or characterizing optimized threshold particle counting. 
         FIG. 2D  illustrates a system for measuring droplet size distributions produced by droplet formers. 
         FIGS. 3  A-C illustrate pneumatic, concentric and flow focusing embodiments of a nebulizer component of the apparatus of the invention. 
         FIG. 4  is a cross sectional view of an embodiment of a plate impactor component which is used in an embodiment of the apparatus of the invention. 
         FIG. 5  is a cross sectional view of an embodiment of a virtual impactor which is used in another embodiment of the apparatus of the invention. 
         FIG. 6  is a diagram showing impactor efficiency. 
         FIG. 7  illustrates an embodiment of a vibrating orifice generator used in an embodiment of the apparatus of the invention. 
         FIG. 8  is a sectional side elevational view of an embodiment of the system of the present invention including a combination nebulizer-impactor. 
         FIG. 9  is a sectional view of the combination nebulizer-impactor. 
         FIG. 10  is an enlarged view showing a portion of the nebulizer-impactor of  FIGS. 8 and 9 . 
         FIG. 11  illustrates an embodiment of a condensation particle counter used in an embodiment of the apparatus of the invention. 
         FIG. 12  is a graph of droplet size distributions produced by various combinations of nebulizers with impactors. 
         FIG. 13  is a graph of the cumulative particle concentration over time using a nebulizer-impactor combination D. 
         FIG. 14  is a graph of particle cumulative concentration versus time for detection of particles in ultra-pure water (UPW), with respect to 30 nm polystyrene latex (PSL) particles. 
         FIG. 15  is a graph of particle cumulative concentration versus time for detection of particles in ultra pure water (UPW) with respect to 22 nm silica particles. 
         FIG. 16  is a graph of differential concentration versus particle size, which shows the ability to size 30 nm particle PSL. 
         FIG. 17  is a graph of CMP slurry PSD measured using a Combination D apparatus with an SMPS detector. 
         FIG. 18  compares an embodiment of the method of the invention with DLS. 
         FIG. 19  shows the change in slurry PSD over time during handling via a graph of differential number concentration versus particle diameter. 
         FIG. 20  shows particle concentrations of particles upstream (feed) and downstream (filtrate) of a test filter. 
         FIG. 21  is a graph of percentage retention of a filter versus particle diameter, which demonstrates retention of the filter as a function of particle size. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention provides a method and apparatus for determining the size distribution and concentration of particles in a liquid. 
     A. Methods of the Invention. 
     The method involves (a) forming droplets, for example via aerosolization, from a liquid sample to be analyzed, (b) isolating small droplets from the droplets, for example less than 10 um in size, (c) drying the droplets to remove the liquid, for example via evaporation, and (d) counting the residual particles. 
     Importantly, the aerosol droplets isolated are small and uniformly sized, less than 10 um and preferably a median size less than 1 um. The droplets must be small and uniformly sized because any dissolved material in the droplet will form a “residue” particle as a result of drying. If the residue particle is large enough it will be detected by the analyzer and interfere with the measurement of true, non-dissolved, particles. 
     The size of a residue particle resulting from evaporation of a liquid droplet can be determined from the concentration of the non-volatile residue in the droplet using equation 1 where d s  is the size of the final residue particle, d d  is the size of the droplet diameter and F v  is the volume fraction of non-volatile residue in the droplet:
 
 d   s   =d   d ( F   v ) 1/3   (1)
 
     If the density of the non-volatile residue in the droplet is the same as the liquid (1.0 g/cm 3  in the case of water), then F v  is simply the weight concentration of non-volatile residue (C). If the water has a non-volatile concentration of 1 ppb with a density of 1.0, equation 1 above can be used to calculate the minimum droplet size that will yield a 25 nm particle, as follows:
 
 d   d   =d   s ( C ) −1/3 =25 nm (10 −9 ) −1/3 =25 nm (1000)=25 μm
 
Hence, the residue in droplets smaller than 25 μm will not be detected by a 25 nm CPC, if the CPC has a very sharp size cutoff.
 
     The required small, uniformly sized droplets required by the present invention may be generated by firstly making droplets of diverse sizes and secondly removing large droplets. Alternatively, the desired droplets may be made in a single step. An example of the former embodiment of the method is implemented by generating droplets by a compressed air nebulizer or an ultrasonic nebulizer and then removing large droplets by directing them to an impaction surface such as a plate impactor or a virtual impactor. An example of generating small, uniform droplets directly is by way of a vibrating orifice aerosol generator. After the droplets are formed, liquid in the droplets is removed before the droplets collide or coalesce. Liquid removal may be accomplished by heating to dry via dilution air, heated air, or heating the liquid. It may also be accomplished by evaporation. And after drying to isolate the particles, the particles are counted and/or sized by OPC, CPC, SMPS or other instruments. 
     Thus, referring to  FIG. 1 , a flow chart of a basic embodiment of the method  10  of the invention involves the steps of, providing  11  a liquid sample, forming  12  very small, uniformly sized droplets, via an aerosol, from the liquid sample to be analyzed, drying  13 , via evaporating, the droplets in the aerosol, and counting  14  the residual particles. Variants of this embodiment of the method are discussed above. 
     B. Apparatus of the Invention. 
     Referring to  FIG. 2A , one embodiment of the apparatus  20  of the present invention comprises means  22  for forming droplets of diverse sizes connected to a sample input  21 . Means  23  for removing large droplets is communicatively connected to the means  22  for forming droplets. An example of droplet former  22  is a compressed air nebulizer, an ultrasonic nebulizer, or a flow-focusing nebulizer, examples of which are shown in  FIGS. 3A-3C . An example of means  23  for isolating small uniformly sized droplets is a plate impactor or a virtual impactor (examples of which are shown in  FIGS. 4 and 5 ) having an impaction surface at which an aerosol stream output by the nebulizer  22  is directed and which removes large droplets. After the desired droplets are formed, liquid in the droplets is removed before the droplets collide or coalesce by liquid removal means  24 . Examples of such means include a stream of dilution air, heated air, or a liquid heater. Drying may also be accomplished by a fast evaporator. A particle analyzer  25  is communicatively connected to the liquid remover  24 . Examples of such an analyzer for counting and/or sizing includes an OPC, CPC, SMPS or other instrument or combination of instruments. Exemplary nebulizers, impactors and analyzers are described in detail below. 
     Referring to  FIG. 2B , another embodiment of the apparatus  30  of the present invention comprises means  32  for making droplets of a small and uniform size connected to a sample input  31 . An example of such means is a vibrating orifice aerosol generator, an example of which is described in detail below. After the desired droplets are formed, liquid in the droplets is removed before the droplets collide or coalesce by liquid removal means  33 . Examples of such means include a stream of dilution air, heated air, or a liquid heater. Drying may also be accomplished by a fast evaporator. A particle analyzer  34  is communicatively connected to the liquid remover  33 . Examples of such analyzer for counting and/or sizing includes an OPC, CPC, SMPS or other instrument or combination of instruments. 
     Testing of threshold particle counting of embodiments of the apparatus of the invention may be performed using the system  100  shown in  FIG. 2C . In this system  100 , particles of various types and sizes, for example a high purity liquid or a colloidal suspension, are injected into a flowing stream of UPW (input  110 ). Concentrations of the injected particles are measured using an HSLIS-M50 optical particle counter (OPC)  120  (of Particle Measuring Systems). A portion of the water is sent through a TPC combination nebulizer/impactor  130 . The resulting aerosol is analyzed using three detector embodiments:
         A 20 nm CPC  140     An SMPS  150 , for example capable of measuring aerosol particle size distributions from 5 to 1000 nm.   A dew point sensor  160         

     The dew point sensor  160  is preferably used to measure the water vapor content of the gas containing the nebulized liquid to determine the instrument inspection volume. Under present optimum conditions, the instrument inspection volume is as large as practicable, preferably approximately 50-10,000 μlit/min [&gt;100 μlit/min]. 
     Processing may be performed with monodisperse polystyrene latex (PSL) particles of various sizes, polydisperse PSL, monodisperse silica particles or other particle types. For example, measurements may be made with 30 nm PSL, polydisperse PSL and 22 nm silica. The size distribution of the monodisperse particles tested can also be measured using a NICOMP 380ZLS (of Particle Sizing Systems). This instrument measures size distributions via dynamic light scattering (DLS)—a commonly used technique for measuring the size of small colloidal particles and the technique used by Duke Scientific (PSL bead supplier) to measure sub-50 nm PSL. 
     An apparatus for measuring the size distribution of droplets formed by various droplet forming methods is shown in  FIG. 2D . A test solution  51  is input to a vessel  52 . Solution  51  is output via a gear pump  53  through a filter  54  and into a small overflow vessel  56 . Most of the liquid input to vessel  56  returns to vessel  52 . A small portion of the liquid is sent to droplet former  55 . Droplet former  55  forms an aerosol  57  containing small droplets. The nebulizer  55  is connected to a pressurized gas source  58 , preferably compressed air or N 2 . The gas is filtered, for example via a Wafergard filter  59 . Various droplet former  55  embodiments are discussed below, 
     The aerosol  57  is input by the droplet former  55  to a drying chamber  70 . The drying chamber  70  is an elongated structure with input and output ends, a predetermined length and a predetermined horizontal dimension. The drying chamber  70  input end is connected to a source of room air via a pump  71 . Air is preferably filtered, for example via a Millipore 0.22 micron Hydrophobic Millipak filter  72 . The droplet former  55  is disposed at a predetermined location on the drying chamber  70 . A scanning mobility particle sizer (SMPS)  33  is disposed at a predetermined location on the drying chamber  70 , a predetermined distance “L” from the nebulizer  55 . A vacuum pump  74  is connected to the SMPS  33 . The pump  74  operates at about 1.54 liters per minute. A thermohygrometer  75 , for example a DigiSense meter is disposed at the output end of the drying chamber  70 , a predetermined distance “P′” further downstream from the SMPS  33 . 
     Referring to  FIG. 3A , an example pneumatic nebulizer  200  which may be used to create initial droplets is disclosed. In a typical pneumatic nebulizer  200  with vent  206 , compressed air exits from a small orifice  201  at high velocity creating a low pressure in the exit region. The low pressure causes liquid to be drawn into the airstream from a second tube  203  from liquid reservoir  202 . The high velocity air causes the liquid to accelerate and break into droplets. The high velocity spray  204  is directed toward an impaction surface where the largest droplets are removed and an aerosol  205  is output. 
     Commercially available nebulizers typically generate aerosols with droplets whose size is log-normally distributed. Median droplet sizes are typically 0.5-5.0 μm. The geometric standard deviation is typically ˜2.0. The large geometric standard deviation means that the nebulizers generate a significant number of large droplets. For example, approximately 0.0003% of the droplets from a nebulizer producing an aerosol with a median droplet size of 1.0 μm and geometric standard deviation of 2.0 would be larger than 25 μm. This is an unacceptable number of large droplets for the applications described above. Examples of commercially available pneumatic nebulizers include Laskin nebulizer, Babington nebulizer, Cross-flow nebulizer, and Pre-filming nebulizer. Referring to  FIGS. 3B and 3C , known concentric  210  and flow focusing pneumatic  220  nebulizers might also be used. Ultrasonic generators are also useable for generating small droplets, but less preferred than nebulizers. 
     As was discussed above, large droplets can be removed from the aerosol using either a plate impactor  300 , shown in  FIG. 4 , or a virtual impactor  350  as shown in  FIG. 5 . In the plate impactor, the plate deflects the aerosol flow to follow an abrupt 90° bend. Droplets with sufficient inertia deviate from the flow stream, impact on the plate, and are removed from the gas stream. A virtual impactor is similar to a plate impactor except that the droplets are impacted into a quiescent region where they are withdrawn from the aerosol by a small secondary flow. 
     The effectiveness of impactors for removing particles is related to the Stokes number or impaction parameter. The Stokes number (S tk ) is proportional to the square of the droplet size as shown in equation 2 where ρ p  is the droplet density, U is the nozzle velocity, η is the gas viscosity, and D j  is the nozzle diameter.
 
 S   tk =(ρ p   d   p   2   U )/(9η D   j )  (2)
 
     Impactors can be designed with sharp efficiency curves. An impactor designed to remove 50% of the droplets &gt;10 μm should remove virtually all droplets &gt;25 μm. An example of a typical impactor efficiency curve is shown in  FIG. 6 . 
     Another approach to generating an aerosol with small droplets is through the use of a vibrating orifice aerosol generator  400 . Referring to  FIG. 7 , these generators  400  work by vibrating a liquid at a high frequency as it passes through a small orifice. They produce nearly monodisperse droplets. The size of the droplets generated can be calculated using equation 3 where Q L  is the liquid flow rate and f is the oscillating frequency:
 
 d   d =(6 Q   1   /πf ) 1/3   (3)
 
A generator operating at 2 MHz with a flow rate of 0.02 ml/min would produce 10 μm droplets.
 
     A preferred approach involves a system including a combination nebulizer-impactor  450 . Referring to  FIGS. 8-10 , an input conduit  428  transfers fluid to a pneumatic nebulizer portion of the system. The nebulizer also receives air, nitrogen or another gas under pressure from a pressurized gas source through conduit  460 . Within nebulizer  450 , the liquid sample and compressed gas are merged to generate an aerosol including droplets of the liquid sample suspended in the gas. 
     Nebulizer  450  includes a reservoir  468  in fluid communication with the merger zone. The reservoir collects most of the liquid supplied through the input conduit  428 , i.e. the liquid not used to form the aerosol droplets. 
     The inclined orientation shown is advantageous for liquid drainage and evacuation, although not critical. A housing of the nebulizer has several integrally coupled sections, including a stainless steel housing section  472  that encloses merger zone  448 , a steel housing section  474  forming the aerosol conditioning zone, and a housing section  476  providing the reservoir. Housing section  472  supports a fitting  478  for receiving the air or other compressed gas from conduit  460 . This housing section also supports an impactor  480 , through a threaded engagement that permits adjustment of the axial spacing between impactor  480  and merger zone  448 . 
     With reference to  FIG. 9 , housing section  472  further supports a thermoelectric device  482  that functions to maintain a stable temperature of about 30.degree. C. in the vicinity of merger zone  448 . More particularly, the thermoelectric device extracts heat from housing section  472  and transfers it to a heat sink  484 . The thermoelectric device also may function as a heater for the nebulizer. The constant temperature promotes consistent droplet formation. Housing section  472  further supports bulkhead fitting  446 , which secures an input conduit  428  used to transfer the sample liquid to merger zone  448 . 
     As best seen in  FIG. 10 , merger zone  448  takes the form of a cylindrical chamber in a Teflon orifice housing  473 . A sapphire orifice plate  486  defines an entrance or primary orifice to receive pressurized gas into the chamber from conduit  460 . A sapphire orifice plate  488  defines an exit or secondary orifice through which the merged liquid and gas leave the chamber. In addition, a liquid receiving entrance  490  conducts the sample liquid into the chamber. 
     In one suitable version of nebulizer  450 , primary orifice  486  has a diameter of 0.006 inches, and secondary orifice  488  has a diameter of 0.008 inches. The chamber has a diameter of 0.020 inches, and an axial length, i.e. space in between orifice plates  486  and  488 , of 0.020 inches. 
     More generally, the secondary orifice diameter is larger than the primary orifice diameter, yet less than the diameter of the cylindrical chamber. As compared to prior devices in which there is no secondary orifice and the chamber is simply open at the exit end, there is a back pressure due to the secondary orifice which increases the feed pressure to the merger zone and results in a higher kinetic energy mixing of the liquid and compressed gas. This advantageously results in smaller sample liquid droplets in the aerosol leaving the merger zone. 
     As the size of the secondary orifice is reduced, the droplet size is reduced and the back pressure is increased. When the sample liquid is water, it has been found satisfactory to form the secondary orifice and the primary orifice at a diameter ratio of two to one as indicated by the diameters given above. For a sample liquid with a boiling point lower than water, the preferred diameter ratio is closer to one, yet the secondary orifice remains larger than the primary orifice. 
     The higher energy in the merger zone more effectively breaks up the liquid. The secondary orifice also appears to improve the efficiency of the impactor downstream. The ratios of primary and secondary orifice diameters can be selected to vary the pressure at the liquid entrance to the merger zone, relative to atmospheric pressure. Depending on the diameter ratio, air inlet pressure and liquid flow rate the liquid pressure can be adjusted from below atmospheric pressure to a pressure nearly equal to the inlet air pressure. 
     As seen in  FIG. 10 , impactor  480  is disposed coaxially with merger zone  448 , spaced apart in the axial direction from orifice plate  488 . The impactor cooperates with housing section  472  to form a thin, somewhat hemispherical path to accommodate the flow of air and droplets beyond the merger zone. The smaller droplets tend to follow the air flow, while the larger droplets tend to collide with impactor  480  and are removed from the aerosol stream. Thus, the aerosol moving into conditioning zone  462 , upwardly and to the right as viewed in  FIG. 8 , includes only those droplets below a size threshold determined largely by the axial spacing between secondary orifice  488  and impactor  480 . The size threshold is increased by increasing the axial spacing, and reduced by moving the impactor closer to orifice plate  488 . 
     The droplets impinging upon impactor  480  may remain on the impactor momentarily, but eventually descend to reservoir  468  then drain from the nebulizer. If desired, impactor  480  may be formed of sintered metal to provide a porous structure that more effectively prevents the larger, impacting droplets from interfering with the aerosol flow. 
     A secondary gas may be introduced into nebulizer  450  at a location upstream of the nebulization region. The secondary gas sweeps dead space in the nebulization region resulting in a faster response, reduced axial diffusion, and less smearing of the output due to mixing. 
     As was discussed above in general, once the aerosol is formed, the liquid in the droplets must be evaporated before the droplets have a chance to collide and coalesce. Drying can be accomplished using dilution air, heated air or heating the liquid. 
     Once the liquid is evaporated, the particles in the aerosol can be counted and/or sized by a number of techniques including, but not limited to optical particle counters (OPCs), condensation particle counters (CPCs) and scanning mobility particle sizers (SMPS). OPCs are similar to those used in liquids. They size and count individual particles as they pass through a laser beam. Examples of OPCs include those made by Particle Measuring Systems, RION, Horiba, Particle Sizing Systems, and Hach Ultra. 
     Referring to  FIG. 11 , a CPC  500  is capable of measuring very small particles in aerosols. They act as “particle size amplifiers” in front of a single particle counting optical detector. Particles drawn into the sensor pass through appropriately cooled and heated sections of a wet walled condenser. The differing mass and thermal diffusivities of the molecules of water vapor and air, create a supersaturated region in which the water vapor condenses on to the particles. The liquid droplets containing the particles grow to a few micrometers in diameter which are then detected optically with very high signal-to-noise. CPCs are available that use a number of working fluids including butanol and water. By varying the design conditions, they can have detection limits varying from about 1 to 20 nm. CPCs by themselves do not measure particle size distributions. They simply determine the concentration of particles larger than a size determined by their operating conditions. However, several CPCs with different detection limits can be combined to determine a size distribution. Alternately, a CPC can be combined with an SMPS to determine the size distribution. 
     In one embodiment, the apparatus of the present invention includes the nebulizer/impactor  450  and a CPC  500  with a detection limit preferably between 20 and 30 nm. This embodiment is believed to be best suited for high purity liquid to measure concentration above a defined threshold, but without particle size distribution (PSD) measurement. 
     In another embodiment, the apparatus of the invention includes a nebulizer impactor  450  and a scanning mobility particle sizers (SMPS). This embodiment is believed to be best suited for determining PSD. 
     Although the apparatus and method of the invention has been described in connection with the field of semiconductor device manufacture, it can readily be appreciated that it is not limited solely to such field, and can be used in other fields. 
       FIG. 12  is a graph of droplet size distributions (differential concentration vs. droplet diameter measured in um) produced by various combinations of nebulizers with impactors. Differential concentration is measured in d (#/cm 3 ) per d log (D P ). The graph includes lines illustrating fits of the PSD to a log-normal distribution. The droplet size distributions were measured by forming an aerosol from a sucrose solution, drying the droplets, measuring the residue PSD and calculating the droplet PSD using the equations above. The graph shows that combination D has the best distribution in that it has the smallest and most uniform droplets, and virtually no droplets are larger than 10 um. 
       FIG. 13  is a graph of the cumulative particle concentration over time (in hours) measured using the nebulizer-impactor combination D. Cumulative particle concentration is measured in #/ml. greater than or equal to 20 nm. The detector is a CPC with a 20 nm detection limit. Particle detection is in ultra purified water (UPW) containing 0.4 ppb non-volatile residue (NVR). The graph shows a detection limit of approximately 8000/ml. The limit would be lower in water with lower residue content.  FIGS. 14 and 15  are graphs of particle cumulative concentration versus time (in minutes) for detection of particles in ultra pure water (UPW).  FIG. 15  is for 22 nm silica particles and  FIG. 14  is for 30 nm polystyrene latex (PSL) particles. These graphs show the detection response of combination D (20 nm) CPC to low concentration of the two particle types.  FIG. 16  is a graph of differential residual concentration (d (nm 3 /cm 3 )/d log (D p )) versus particle size (in nm) which shows the ability to size 30 nm particle PSL. One sizing was conducted with a combination D apparatus with an SMPS detector. Another was conducted with a dynamic light scattering (DLS) instrument, more particularly with a NICOMP 380ZLS made by Particle Sizing Systems, Santa Barbara, Calif. The comparison shows generally good agreement. The combination D apparatus with SMPS permits measurement of actual number concentration. In contrast, DLS only provides relative concentrations. The combination D apparatus also provides a more detailed measurement of PSD. DLS on the other hand assumes that the particles are log-normally distributed.  FIG. 17  is a graph of differential number concentration vs. particle diameter which measures CMP slurry PSD using a Combination D apparatus with an SMPS detector. The graph shows good separation between residue and slurry particles. And  FIG. 18  compares this method with DLS (using the NICOMP 380ZLS). The graph shows good agreement between the two processes. The differential number concentration determined via this method is normalized to a maximum concentration of one since DLS only gives relative concentrations.  FIG. 19  shows the change in slurry PSD over time during handling via a graph of differential number concentration versus particle diameter. Successive times 1-9 are graphed. The number of smaller particle decreases over time while the number of large (i.e. greater than approximately 250 nm) increases. This indicates that particle agglomeration is occurring due to handling. This is an example of usefulness of the method of the invention.  FIGS. 20 and 21  provide measurements of filter retention. Filters were challenged with polydisperse mixtures of PSL particles ranging from 20 to 500 nm in diameter. The graph (cumulative concentration in #/ml vs. particle diameter in nm) in  FIG. 20  shows concentrations of particles upstream (feed) and downstream (filtrate) of the filter. The graph in  FIG. 21  (percentage retention vs. particle diameter (nm)) shows retention of the filter as a function of particle size. 
     The embodiments above are chosen, described and illustrated so that persons skilled in the art will be able to understand the invention and the manner and process of making and using it. The descriptions and the accompanying drawings should be interpreted in the illustrative and not the exhaustive or limited sense. The invention is not intended to be limited to the exact forms disclosed. While the application attempts to disclose all of the embodiments of the invention that are reasonably foreseeable, there may be unforeseeable insubstantial modifications that remain as equivalents. It should be understood by persons skilled in the art that there may be other embodiments than those disclosed which fall within the scope of the invention as defined by the claims. Where a claim, if any, is expressed as a means or step for performing a specified function it is intended that such claim be construed to cover the corresponding structure, material, or acts described in the specification and equivalents thereof, including both structural equivalents and equivalent structures, material-based equivalents and equivalent materials, and act-based equivalents and equivalent acts.