Abstract:
A deposition system is used for depositing particles onto a substrate, such as a wafer in a deposition chamber. The particles are carried in an aerosol that is generated an atomizer that includes an impaction plate for removing large particles before the aerosol is discharged, and which has an output that is provided through a particle classifier to the deposition chamber. Various branches of flow lines are used such that the aerosol that has classified particles in it, is mixed with a clean dry gas prior to discharge into the deposition chamber, and selectively the aerosol can be directed to the deposition chamber without having the particles classified. The lines carrying the aerosol can be initially connected to a vacuum source that will quickly draw the aerosol closely adjacent to the deposition chamber to avoid delays between deposition cycles.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to a method and apparatus for deposition of particles on surfaces, wherein the particles are provided from an aerosol generation device that regulates the droplet size and concentration provided to the deposition chamber so that precisely sized particles or spheres are deposited on the surface.  
           [0002]    Pneumatic atomizers are often used for generating aerosols containing polystyrene latex (PSL) spheres or particles, as well as other particles, For subsequent deposition on substrates, such as semiconductor wafers. The particles are first suspended in liquid such as deionized water to form a suspension. The suspension is then atomized to form droplets. When the droplets evaporate, the PSL spheres or particles become airborne particles. The generation rate of PSL spheres or particles is a function of droplet generation rate of the atomizer and the probability for a droplet to contain PSL spheres or particles.  
           [0003]    The droplets produced by a pneumatic atomizer normally have a broad size distribution ranging from less than 0.1 μm to larger than 10 μm. Large droplets have a high probability to contain more than one PSL sphere or particle. If a droplet contains more than one PSL sphere or particle, it is called a multiplet. Multiplets provide more PSL particles than those wanted.  
           [0004]    A droplet that does not contain any particles is called an empty droplet. When an empty droplet evaporates, it forms a residue particle resulted from the precipitation of nonvolatile impurities dissolved in the atomizing solution. For example, to prepare a PSL suspension, surfactant is often used to keep suspended PSL spheres from coagulating. The surfactant is one of the sources for residue particles. The size of residue particles depends on the size of the droplets and the concentration of nonvolatile impurities in the atomizing solution. At a given concentration of the nonvolatile impurity, the residue particle size is linearly proportional to the droplet size.  
           [0005]    For PSL or particle deposition, the multiplets and the residue particles are always unwanted. Special atomizers will minimize the formation of multiplets and the size of residue particles by removing large size droplets.  
         SUMMARY OF THE INVENTION  
         [0006]    The present invention relates to a system for depositing particles on surfaces, in particular semiconductor wafers. The invention insures that there is a minimal amount of unwanted material deposited on the wafer, and that each droplet of the aerosol contains only one sphere or particle. Residues are minimized, and the deposit is uniformly made.  
           [0007]    The present invention, in one aspect, provides for an atomizer that will atomize droplets that are only within a particular size range, and will insure that the droplets from the atomizer are of size so they will contain only one particle of the desired material that is going to be deposited. In this way, empty droplets are avoided, and multiplets, that is, a droplet that contains more than one particle, are also avoided.  
           [0008]    A differential mobility analyzer, which can be adjusted to emit only the particles that are of proper size, is utilized for insuring one size particle.  
           [0009]    Various forms of devices are included for checking the density of the particles in the aerosol and the flow rate. The flow lines permit adding clean gas to the flow of the aerosol as needed, and a pre-deposition sequence permits the aerosol flow to be established at a junction adjacent to the deposition chamber and then switched to the deposition chamber. The procedure reduces the time between deposition cycles. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1 is a schematic representation of a wafer deposition system made according to the present invention;  
         [0011]    [0011]FIG. 2 is a flow diagram illustrating the control of the gas that is used in the atomization process;  
         [0012]    [0012]FIG. 3 is a schematic sectional view of a atomizer arrangement used with the present invention;  
         [0013]    [0013]FIG. 4 is a vertical sectional view of a differential mobility analyzer used in the present invention;  
         [0014]    [0014]FIG. 5 is a schematic diagram showing two differential mobility analyzers used to broaden the size range of particles that can be processed;  
         [0015]    [0015]FIG. 6 is a schematic diagram of the aerosol flow lines adjacent the deposition chamber; and  
         [0016]    [0016]FIG. 7 is a schematic representation of connections of a particle counter. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0017]    Referring to FIG. 1, a schematic diagram of an entire wafer deposition system is illustrated generally at  10 , and includes an aerosol generator or atomizer section  11 , which will provide an aerosol along a line  12  to a differential mobility analyzer  13 , that classifies the aerosol particles according to size and passes the classified particles along a line system  14  to a deposition chamber illustrated generally at  15 . Chamber  15  is used for depositing particles carried in the aerosol onto a wafer. Deposition chambers are well known in the art. A fluid flow through the lines is provided by the positive pressure at the aerosol generator and by a vacuum pump. Vacuum pump  16  is not used to evacuate the deposition chamber, but is used to establish initial flow in the lines and through a particle counter.  
         [0018]    The individual sections have valves, flow controllers, pressure regulators, and the like as will be explained in connection with the individual sections.  
         [0019]    [0019]FIG. 2 illustrates the atomizer section  11 . A source of clean dry gas  17  provides the gas through a pressure regulator  18 , a mass flow controller  19 , and a three way valve  21  to an atomizer indicated generally at  20 . The mass flow controller  19  controls the mass flow from the pressure regulator  18  to the atomizer  20  The flows are balanced and clean gas can be added to and mixed with the atomizer output.  
         [0020]    As shown in FIG. 3, atomizer  20  has a body  23  with an air or gas inlet passageway  23 A from the flow controller  18 . The flow passes through an orifice  23 D into an atomization chamber or nozzle  23 B into which an atomizing liquid  23 L from a container is drawn. The liquid is broken up into droplets in chamber or nozzle  23 B. An impaction plate  22  is installed adjacent to but spaced from the outlet of the atomizer nozzle  23 B to remove large droplets by impaction. The impaction plate  22  can be an annular band or wall, if desired.  
         [0021]    Three parameters for determining the output droplet size and volume are dimensions of the orifice or passageway  23 D between inlet passageway  23 A and atomizer nozzle  23 B, the diameter of the nozzle  23 B, and the distance from the outlet opening of nozzle  23 B and the impaction plate  22 . The diameter of orifice  23 D is identified as D 1  and indicated by arrows  24 , the diameter of the output nozzle  23 B is identified as D 2  and indicated by arrows  26 . The distance from the nozzle  23 B outlet to the impaction plate surface is identified as D 3  and indicated by arrows  28 . Atomizing orifice  23 D controls the total atomizing gas flow. When D 1  is constant, reducing dimension D 2 , the outlet diameter of the nozzle  23 B, and reducing dimension D 3 , the distance from the nozzle outlet to the impaction plate surface, will result in a smaller output droplet size. By selectively changing D 2  and D 3 , the size of the droplets produced by the atomizer can be regulated. The droplet size is selected so each droplet will contain one PSL particle. The PSL particles are also regulated in size, and the goal of no empty droplets and no multiplets can be achieved. With different types of particles, this goal also can be achieved by appropriate sizing of the orifice  23 D, the atomizing nozzle  23 B, and the distance from the nozzle to the impaction plate.  
         [0022]    The aerosol produced from the atomizer  20  of FIG. 3, and other atomizers, consists of droplets carried in a saturated gas, usually air. One way to evaporate the droplets is to mix the aerosol droplets with dry gas or air. Referring to FIG. 2, the clean dry gas from source  17  and mass flow controller  19  splits into two streams at a junction  30 . The atomizing gas flows into the 3-way valve  21 . A mixing gas flow is diverted from a junction  30  along a line  32 , through a mixing flow control comprising an orifice  34  One port of the 3-way valve  21  is selectively connected to the inlet of atomizer  20  and the other port of valve  27  is selectively connected to a bypass line  38  which has balancing flow control orifice  40 . The output lines from the atomizer, line  32 , and line  38  join at junction  20 J.  
         [0023]    During aerosol generation, the 3-way valve  21  is connected to the inlet passage  23 A of the atomizer  20 , producing aerosol droplets. The aerosol droplets then mix with a controlled volume of clean dry air/gas from the mixing flow control orifice  34 . If the atomizer  20  is to be shut off, the valve  21  directs flow through balancing flow control orifice  40 .  
         [0024]    The flow from 3-way valve  21  through the inlet passage  23 A, orifice  23 D and nozzle  23 B of the atomizer  20  produces the aerosol droplets by aspirating liquid containing PSL particles (or other particles) from the liquid and particle source  23 L. The aerosol droplets then mix with clean dry air or other gas provided at a junction  20 J from the mixing flow control. After mixing, the droplets will evaporate, forming an aerosol of PSL spheres or particles for deposition. One way to control the three flows, that is, the aerosol flow, the mixing flow and balancing flow is using properly sized orifices. The control orifice  34  for the mixing flow, the orifice  23 D for the atomizing flow and the orifice  40  for the balancing flow are sized such that at a given pressure of the clean dry gas or air, the total flow through the aerosol generator to line  41  is a constant regardless of whether the 3-way valve  21  provides the input flow to the atomizer for atomizing liquid or to the balancing flow control orifice. The atomizing flow is shut off when the valve  21  is moved to provide flow to the line  38 .  
         [0025]    The output from the atomizer in line  41 , goes through a 3-way valve  42 . The valve  42  can divert the aerosol along a line  42 A that bypasses the size classification. The normal operating position of valve  42  will transmit the aerosol to a junction  41 J (see FIG. 1) in the line  12  where the desired flow goes to the differential mobility analyzer  13 .  
         [0026]    The line  12  has a flow control orifice  44  in the line, as well as a charge neutralizer  46 , which will de-ionize the aerosol, and reduce electrical charges from the particles. The line  12  is branched at junction  41 J to line  48 , that passes through a filter  50 , and a flow control restriction  52 . The flow control restriction  52  is illustrated as an orifice, but also could be a mass flow controller. The flow restriction will control the volume of the aerosol that is diverted through the line  48  and through filter  50  as a function of the total flow and the flow provided to the DMA  13 . A pressure sensor  54  is used for sensing the pressure in the line  48 , and keeping it regulated appropriately, and a temperature sensor  56  is also utilized. These parameters are utilized as feedback for controlling the inputs to the DMA. The line  48  is connected to the sheath flow input to the differential mobility analyzer, and provides what is called the DMA sheath flow. The filter  50  removes most of the particles in the aerosol, so the sheath flow is essentially a clean gas.  
         [0027]    The aerosol in the line  12  is injected into the center of the DMA. The DMA will discharge only particles that are of a desired size. The DMA is used to insure that the particles that are to be provided to the deposition chamber will be only one size or monodisperse.  
         [0028]    The differential mobility analyzer (DMA)  13  is shown in detail in FIG. 4, and it operates to classify particles so that they are monodisperse particles. The DMA  13  comprises a tubular housing  62  through which the divided flow from the atomizer  20  passes and includes the sheath flow from line  48  as mentioned, as well as the aerosol flow from line  12 . The aerosol flow, which is indicated by the block  64  in FIG. 4 enters ports  66  in the housing  62  and flows down through an annular passageway  68  formed between the inner surface of housing  62  and a flow distributor  72 , which is a sleeve spaced from the outer housing to provide an aerosol flow passageway, and surrounds a central electrode  70 , which is a tubular electrode. The aerosol flow thus surrounds and is spaced from the tubular central electrode  70 . The aerosol flows down along the outside of the flow distributor sleeve  72 , so that it stays along the inside surface of the housing  62 . The sheath flow, indicated by block  65 , from line  12  is introduced through a port  74 , and flows down through a central passageway  77  of an insulator sleeve  76  that has a high voltage electrode  78  which is connected to a source of high voltage and which extends through the central passageway, and connects to the tubular high voltage electrode  70 .  
         [0029]    As the sheath flows down through the passageway  77 , it will be discharged into the interior of the flow distributor  72  and flow down along the surfaces of the tubular electrode  70  to provide a sheath of clean air surrounding the electrode. The aerosol particles carrying a low level of electrical charge, as they move from the inlet end  66  of the DMA housing  62  to the outlet, the voltage on the electrode  70  is set so the correct size of particles will be attracted to enter an opening shown at  82  in the side wall of the electrode, and then discharge out through a central passageway  84  in an end piece  86  of the tubular electrode  70 . The particles of the selected size discharge out through a line  88 . The output of the DMA is a monodispersed aerosol, that is, an aerosol with only one size particle. The voltage from the source  80  controls the size of the particles that will enter the opening  82 , and at a set voltage only one size will pass through the passageway  84  and the line  88 .  
         [0030]    Excess flow and containing particles that are of a different size from that which will pass through the opening  82 , are carried out through an excess flow passageway  90 , and through a filter  90 A, a flow controller  90 B and a line  90 C to a desired location.  
         [0031]    The total flow from the aerosol generator  11  can be maintained at a set level, the flow from one outlet of valve  42  is split into two flow streams, one for the DMA sheath flow and the other comprising a polydisperse aerosol flow to be size-classified by the DMA. The ratio of the DMA sheath flow rate to polydisperse aerosol flow rate is controlled by the two flow restrictions  44  and  52  shown in FIG. 1. All the particles in the DMA sheath flow are removed by filter  50  (which can have two sections) prior to the flow restriction or flow control device  52 . The flow restriction or flow control device  52  for the sheath flow can be an orifice flow restriction or a flow controller such as a mass flow controller. The polydisperse aerosol flow in line  12  cannot be satisfactorily controlled by a mass flow controller since the flow carries a high concentration of particles, some of which would be removed by a mass flow controller.  
         [0032]    The flow restriction device  34  is an orifice or similar device that will restrict the aerosol flow without loss of particles. The ratio of the DMA sheath flow rate to the polydisperse aerosol flow rate is fixed if orifices are used for controlling both DMA sheath flow and the polydisperse flow. The ratio can be adjusted by adjusting the sheath flow rate with flow control device  52  if it is a flow controller. The total flow through the DMA is kept constant, and the output particle size is controlled by the voltage of source  80 .  
         [0033]    The DMA monodisperse aerosol output flow from DMA  13  that is directed to line  14  is controlled by orifice  92 . The DMA excess flow can be controlled by an orifice  90 B or a flow controller. When using an orifice to control both flows from the DMA, the two orifices are properly sized to keep a constant ratio of the flow rates in lines  88  and  90 C. When the DMA excess flow in line in  90 C is controlled by a flow controller, the ratio of the two flow rates, that is the ratio of flows in lines  88  and  90 C, can be adjusted by adjusting the DMA excess flow with a flow controller replacing orifice  90 B.  
         [0034]    As shown in FIG. 5 two differential mobility analyzers are provided in a modified embodiment of the invention to widen the size range of particles that can be provided in the monodisperse flow to the deposition chamber. DMA  13  has a long housing and flow path and can classify particles in a size range from 0.10 to 2.0 μm. An additional short housing DMA  136  will classify a range of particles from 0.01 to 0.3 μm. The DMA  136  operates in the same manner as DMA  13 , except the parts are made to suit the smaller size particles. When combined, the Dual-DMA system covers a size range from 0.01 to 2.0 μm.  
         [0035]    To accommodate two DMA&#39;s, a 3-way valve  137  is placed in line  48  downstream from flow restriction  52 . A line  138  is connected to one output of valve  137  and carries the sheath flow to DMA  136  when valve  137  is in position to connect line  48  to line  138 . The polydisperse aerosol line  12  is branched with a 3-way valve  140  and a connected line  142  to the aerosol input of the DMA  136 . The DMA  136  is constructed as shown for the DMA  13 , but the different length and other known design dimensions results in being operable for the different range of particle sizes.  
         [0036]    The monodisperse outlet line  144  of DMA  136  is connected through a 3-way valve  146  to the output line  88  of DMA  13 , upstream from the flow restriction  92 . The excess flow from DMA  136  is discharged through a filter  147  and line  148 . The excess flow can be discharged as desired. The 3-way valves  137 ,  140  and  146  can be simultaneously operated by a central controller  151  when the output from atomizer  11  is providing particles in the range for the respective DMA. The controller  151  is used to control all the valve flow controllers, pressure regulators and the like. Feedback from the pressure sensors, temperature sensors and flow sensor are used by central controller  151  to provide the proper adjustments.  
         [0037]    As shown in FIG. 1, after the monodisperse flow passes through flow restrictor or flow control orifice  92 , the monodispersed aerosol flow can be mixed at a junction  91  with a clean gas or air, that is fed from a junction  97  on the output line  18 A of regulator  18  through branch line  94  and  95 . Line  94  has an orifice  96 , a flow controller  98  and a filter  100  for regulating flow and for removing any particles. The particle carrying gas, mixed with the dry clean gas to achieve the correct particle density in the flow moves along a line  102 . A further flow control restrictor  104  is provided. A first  3 way valve is provided to selectively direct the flow to a waste line when deposition is not desired.  
         [0038]    A second 3-way valve  108  in line  102  is used to direct the aerosol flow either to a spot deposition nozzle in a deposition chamber  110  along line  111  or to a deposition showerhead along a line  109 . The deposition chamber  110  can be made as desired. The aerosol is then deposited onto a wafer in the chamber with the exhaust going through a filter  118 . The flow through the deposition chamber  110  is determined by pressure differentials in the lines used.  
         [0039]    If desired, the flow from the output of the valve  108  along lines  111  and  109  can be drawn directly to the vacuum pump  16  through a filter  114 , and an on/off valve  116 . When valve  116  is open, flow will pass through a flow restrictor  119  and then to the low pressure side of the vacuum pump  16 . Additional filters can be provided as desired. The line  109  from the 3-way valve  108  is coupled into a line  120  which, as shown, is also connected to the output of a pressure regulator  18  through line  95 , a flow restrictor  126 , a flow controller  122 , and a filter  124 . An on/off valve  128  provides a bypass around the flow controller  122 . Flow restrictor  126  remains in the flow lines regardless of whether valve  128  is on or off.  
         [0040]    The flow from line  95  also can be sent through flow controller  122  as a purge flow to purge the deposition chamber with clean dry air or gas.  
         [0041]    If desired, the output aerosol from the atomizer  20  can be diverted by valve  42  along the line  42 A to line  120  and thus to the deposition chamber for direct deposition, without passing the aerosol through the DMA. The direct deposition function is normally used for depositing large size PSL particles (500-4000 nm). In this case, the residue particles are not of concern since they are normally much smaller. Typically, residue particles are smaller than 30 to 50 nm under normal operating conditions of atomizers presently available.  
         [0042]    Another feature of the present invention is shown in FIG. 6. The response time for the deposition system  10  can be reduced by reducing the time lag for introducing the aerosol from lines or passages  111  or  109  to deposition chamber  110 . Prior to deposition, the aerosol is drawn to the close vicinity of the deposition chamber  110  by vacuum from vacuum pump  16  as controlled by on/off valve  116 . The vacuum pump  16  is designed such that it will provide a flow that is slightly higher than the required deposition aerosol flow. When the valve  116  is turned on (open), the aerosol from any one of the lines  109 ,  111 , or  120  will be pulled from valve  108  and line  109  or line  111  to the exhaust by the vacuum pump  16 . In addition, since the vacuum flow rate is slightly higher than the desired deposition aerosol flow, there will be a small reverse flow from the deposition chamber  110 , via the deposition nozzle  115 B or the deposition showerhead  115 A (see FIG. 6), to the vacuum pump  16  when the valve  116  is turned on. This flow will remove contaminates from the deposition chamber and will cause the respective line  109  or  111  (depending on the setting of valve  108 ) or from line  120  if it is being used, to fill with the aerosol down to the junction with lines  109 A and  111 A, connecting the main portions of these lines to valve  116 .  
         [0043]    The spot deposition nozzle  115 B is connected to line  111  and is for depositing particles in controlled size spots on a wafer. The deposition showerhead  115 A is connected to line  109  and is for larger area deposition, as is well known.  
         [0044]    After valve  116  has been on sufficiently so the line  109  or  111  is filled with the desired aerosol and the deposition chamber  110  is purged by the reverse flow, the on/off valve  116  will be shut off and the aerosol in either line  109  or line  111  will enter the deposition chamber immediately because of the close coupling of the lines to the chamber and the prefilling of the lines with the correct aerosol. The deposition response time is thus significantly improved by providing the preflow out the vacuum pump  16 .  
         [0045]    After each deposition cycle, the valve  116  for the vacuum control is turned on by central controller  151 . The residual particles in the spot deposition nozzle or in the deposition showerhead after each deposition will, therefore, be sucked to the vacuum source. Cross contamination of particles between depositions is avoided.  
         [0046]    Also as shown in FIGS. 1 and 6, orifice  104  is used for flow measurement in combination with a differential pressure sensor  105  to measure and monitor the deposition flow of mono size aerosol flow in line  102 . During deposition, the deposition flow and aerosol concentration are continuously monitored and the deposition time is dynamically adjusted, based on the measured aerosol concentration and deposition flow rates.  
         [0047]    A particle counter  160  (FIGS. 1 and 7) which is a condensation nuclei counter (CNC) is used to determine aerosol concentration by counting the number of particles that pass through the counter when the flow is held at a standard flow rate. The counter input line  162  is connected to line  102  through line  163  and a valve  164 . The particle concentration can be measured at set intervals or for a set time as each deposition cycle starts. The output line from the counter  160  is connected to vacuum pump  16 .  
         [0048]    As shown in FIG. 7, a flow restriction device or orifice  166  in a bypass line is used to control the CNC bypass flow. The flow restriction device  166  is sized to have the same flow rate as the CNC  160  sampling flow rate. That is, the flow rate through the line  163 , which carries flow from line  102  to the CNC counter  160  or, when the counter is shut off, to flow restriction  166 , is kept as a constant, whether the valve  164  is turned to provide flow to the CNC  160  or turned to provide flow through the flow restriction  166 . The constant bypass flow through the CNC or restriction  166  helps to maintain the stability of the entire deposition system during operation. The particle concentration in the aerosol is determined by the CNC operating at a standard flow rate. An on-off valve  165  can be used to positively stop flow through the CNC  160 .  
         [0049]    The dynamic adjustment of the deposition time parameter is based on the measured aerosol concentration from counter  160  and deposition flow rate signals from restriction  104  and pressure sensor  105 . With proper calibration, a very high deposition count accuracy of achieved. For example, a deposition count accuracy of ±3% is achieved, which is the combination of flow and concentration measurement accuracies.  
         [0050]    The volumetric flow rates of DMA sheath flow, input aerosol flow to the DMA in line  12 , the monodisperse aerosol flow in lines  88  and  102 , and excess flow in line  90  directly affect the sizing accuracy of the DMA. If a DMA is calibrated at a certain temperature and pressure, the DMA may not give an accurate sizing response if it is used in an environment that has a different ambient temperature and/or pressure. Temperature sensor  56  and pressure transducer  54  measure DMA temperature and pressure (ambient temperature and pressure, and air/gas temperature and pressure inside the DMA). The signals from the real-time measurement of temperature and pressure are sent back to the controller  151  for proper compensation to the flow controllers and other variable parameters to ensure the sizing accuracy of the DMA.  
         [0051]    As shown in FIG. 4, the DMA is an instrument that classifies particles according to electric mobility of the particles. It can be described as a cylindrical condenser consisting of a metal rod concentrically located within a metal tube. Polydisperse aerosol and clean sheath air are introduced into the DMA and flow down the annulus between the center electrode and the outer tube as laminar streams. A high DC voltage is applied to the center electrode while the outer tube is grounded. The electric field between the two cylindrical electrodes cause charged particles in the aerosol to deflect across the streamlines to the exit slit near the bottom of the tubular electrode rod. The voltage needed to deflect particles to the output air stream is then related to the electric mobility of the particles. The relation between the particle diameter and the required center rod voltage can be obtained using such known equations. In practical use, the voltage is scanned to find the peak voltage corresponding to the maximum particle concentration in the monodisperse aerosol output stream. The voltage is then used to calculate the corresponding particle size.  
         [0052]    All size particles other than the selected size from the DMA, including residue particles and multiplets of the PSL spheres, are removed by electrostatic separation by the DMA. If the atomizing PSL solution has one PSL peak, the DMA will output the PSL spheres at the peak size. If the PSL solution contains multiple PSL size peaks, the DMA will output the peak size PSLs closest to the size specified by the operator. For example, if four PSL sizes are to be deposited onto a wafer, four containers with solutions which each contain one specific PSL sphere size are provided. The four PSL sphere sizes can be mixed and use the DMA system to output one PSL sphere size at a time for deposition.  
         [0053]    The DMA system using two DMAs covers the size ranges of 100 to 2000 nm. The two DMA systems offer the highest accuracy and resolution in its size range. The low detection limit of the smaller DMA can be extended to 3 nm.  
         [0054]    Classified deposition particle size, that is, using a mono-size aerosol from the output of the DMA, is much preferred for PSL or process particles smaller than 1000 nm. After being neutralized to remove excess charges from atomization in neutralizer  46  the aerosol is received by the DMA and classified by the DMA either by direct classification, or size distribution scan and classification. In the size distribution scan and classification mode, the aerosol from the atomizer is first scanned to determine the aerosol size distribution and then classified for deposition. In this operation mode, only the PSL spheres at the peak size are deposited regardless of the broadness of the original distribution of the PSL spheres in the atomizing solution. The PSL size in this operation mode is referred as the Label Size, which is given by the PSL sphere manufacturer. For creating Absolute Contaminant Standards, this operation mode is most widely used when United States National Institute of Science and Technology (NIST) or NIST traceable PSL spheres are used for deposition.  
         [0055]    The classification-only operation mode is often preferred by experienced users. In this mode, the particle size is referred as the DMA size based on the particle&#39;s electrical mobility. Since the DMA is calibrated using NIST standard PSL spheres, the DMA size is in good agreement with the standard PSL spheres. The DMA has a sizing accuracy of ±2% while PSL spheres from different vendors may have as much as 10% difference in size. The DMA size is, therefore, more accurate than most Label Sizes including some NIST traceable PSL spheres.  
         [0056]    The classification-only mode is also referred as the process particle deposition mode since it is widely used for process particle deposition. In process particle deposition, the original particles in atomization solution normally have a broad size distribution. With the classification mode of the DMA, the output particles for deposition can be any size within the original distribution. In this operation mode, the deposition can be made very fast, for example, up to 30 depositions per hour.  
         [0057]    The chamber  110  has provisions for both spot deposition and full deposition. The spot deposition is useful since it can deposit multiple spots of different sizes on a single wafer. Advantages of using multiple spots include reduction in inspection system calibration time and cost, increase in calibration accuracy, improvement in inspection system performance and ease of monitoring the contamination level of the Wafer.  
         [0058]    Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.