Abstract:
A novel method for analyzing the pulse train resulting from a scanned beam particle monitor is described. The method enhances signal-to-noise ratio and significantly reduces particle false alarm rate. Performed in the time domain, the method filters noise pulses that do not occur at the scanner frequency. The analysis further identifies particle-pulse-envelopes (PPE&#39;s) by performing a forward-looking and backward-looking autocorrelation. Gaussian fits are subsequently applied to identified particle-pulse envelopes, to determine particle characteristics, such as size and speed.

Description:
[0001]    This application claims priority upon a provisional patent application, Application No. 60/318,073, filed Sep. 7, 2001. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This invention relates to the field of semiconductor wafer processing, and more particularly to a method for analyzing the signals resulting from a scanned beam particle monitor.  
         BACKGROUND OF THE INVENTION  
         [0003]    In-situ particle monitoring (ISPM) sensors can provide continuous monitoring of particulate contamination levels during key semiconductor process operations. Based upon light-scattering detection techniques, ISPM sensors are typically installed downstream of the process chamber, such as to a pump-line, and provide real-time measurement of variations in particle concentration and size during wafer processing. However, there are several inherent disadvantages to pump-line installation of a sensor apparatus. First, a particle depositing on a processed wafer cannot be measured with the sensor in the pump-line configuration. Second, and because ISPM sensors depend on various particle transport mechanisms to detect the particles generated upstream in the process chamber, ISPM sensor applications often produce poor correlation with the number of particles that deposit directly on the product wafer surface. In addition, the particle detection volume for ISPM sensors is also limited by the small cross-sectional area of the laser beam which is illuminating the particle (s).  
           [0004]    To improve the correlation between ISPM sensors and the number of particles that deposit on the wafer surface, an advanced above wafer in-situ particle monitoring (hereinafter AWISPM) sensor has been developed. As described in U.S. Pat. No. 5,943,130, the entire contents of which are herein incorporated by reference, an AWISPM sensor is capable of monitoring particulate contamination levels within the process chamber. The sensor incorporates a scanned laser beam to create a large detection volume compared to stationary laser beam systems previously known to ISPM sensor technologies. Since the detection volume is only approximately 4 mm above the wafer surface, the capture rate of the AWISPM sensor is enhanced for particles that will deposit directly upon the wafer surface. The capture rate for particulate contamination is also improved by the larger detection volume, which provides a significant increase in the volumetric sampling rate. The AWISPM sensor provides information regarding the actual count, size, and velocity (speed) of the detected particles.  
           [0005]    Because a scanned laser beam is used to detect the presence of the particle (s), a particle will be detected multiple times as it passes through the measurement volume, if the particle drift velocity is small compared with the laser beam velocity at the sample volume. Laser scanning is accomplished using either a resonant scanner or a rotating polygonal mirror. Either optical device results in a precisely defined period (e.g., frequency) associated with the scanned beam.  
           [0006]    This defined period forms the basis of the present invention, allowing the detection and isolation of particle-pulse-envelopes (PPE&#39;s) from the continuous stream of low level electronic and optical noise (stray light) that is typically included in the observed signal. The principle contribution of the signal-processing algorithm is a substantial decrease in the level of false alarm counts at the minimum particle size detectability limit. Since the data stream is analyzed in the time domain relative to the scan frequency, PPE detection at or near signal-to-noise ratios of one can be accomplished.  
         SUMMARY OF THE INVENTION  
         [0007]    It is a primary object of the present invention to overcome the above-noted deficiencies of the prior art.  
           [0008]    It is another primary object of the present invention to eliminate or substantially reduce the incidence of false negatives occurring in a measurement volume using a scanning light source/detection apparatus.  
           [0009]    Therefore and according to one preferred aspect of the present invention, there is provided a process for processing a scattered light signal to determine the presence of particles in a scanned measurement volume, said method comprising the steps of:  
           [0010]    applying a scanning light beam onto a measurement volume;  
           [0011]    detecting scattered light pulses from particles moving through said measurement volume; and  
           [0012]    processing the detected light pulses, said processing steps including the step of determining the existence of multiple pulses representative of a scanned particle in a time domain by generating a corresponding particle pulse envelope (PPE) in order to segregate particles moving in the measurement volume from noise.  
           [0013]    Preferably, and in practice, the light pulse that is scattered by each scanned particle crossing through the measurement volume is initially detected by a photo-multiplier. The electrical current output from the photo-multiplier is then converted by conventional means into a voltage pulse, which is subsequently amplified such as for example, by means of a high-speed, two stage analog amplifier. A level-triggering device, such as a discriminator, eliminates those signals that do not exceed a minimum threshold value.  
           [0014]    The pulse train exiting the discriminator is analyzed with a peak-detector to determine the maximum value of the pulse. This value, as well as the pulse time stamp (micro-second resolution) is stored, for example, on a computer hard disk or in the memory of a microprocessor, for post processing using a preferred signal-processing technique which is described below.  
           [0015]    The signal processing method of the present invention is comprised of three primary functions: 1) noise removal through analysis in the time domain; 2) identification of particle-pulse envelopes (PPEs) through forward and reverse autocorrelation; and 3) Gaussian fitting to each particle-pulse envelope for particle size and speed estimates. PPE identification through forward and reverse autocorrelation is particularly important since it facilitates identification of singular particle scattering events. Achieving these count statistics is a primary objective of AWISPM sensor measurement.  
           [0016]    An individual pulse can be confirmed as being either signal or noise by determining whether or not it is part of a PPE. This determination is accomplished by looking ahead and behind the pulse (in the time domain) for a second pulse that lags or leads within the scanner frequency.  
           [0017]    After the removal of noise pulses, the signal-processing algorithm begins searching for PPE&#39;s. An autocorrelation calculation in the forward and reverse directions (again in the time domain) is used to identify the beginning and end of an individual PPE. At the start of the PPE, the autocorrelation is high in the forward direction and zero in the reverse autocorrelation. At the end of the envelope, autocorrelation is high in the reverse direction and zero in the forward direction. Through this approach, the beginning and end of a pulse envelope (particle) can be identified.  
           [0018]    After the pulse envelope has been identified, a Gaussian fitting routine is applied. The algorithm begins with an assumed Gaussian mean, width, and amplitude. Each of these values are subsequently iterated to produce minimum error between the peak pulse amplitudes and the value of the Gaussian fit.  
           [0019]    The Gaussian fit is crucial to the determination of particle characteristics, such as size and velocity. Once the fit of an envelope is obtained, the peak amplitude of the Gaussian fit is used to determine particle size while the slope of the Gaussian fit coupled with the diameter of the laser beam at the measurement volume together are used to determine particle velocity.  
           [0020]    According to a preferred version of the invention, it has also been learned that by setting the discrimination level to at least three times and preferably four times the level of background noise will significantly reduce the number of false negatives that are detected which is essential to better determine the existence of single pulse events (e.g., particles which are contacted only a single time by the scanned beam).  
           [0021]    Other objects, features and advantages will become readily apparent from the following Detailed Description which should be read in conjunction with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]    [0022]FIG. 1 is a pictorial representation of a known in situ particle measurement system;  
         [0023]    [0023]FIG. 2 is a pictorial representation of an in situ particle measurement system made in accordance with the present invention;  
         [0024]    [0024]FIG. 3 is a partial sectioned side elevational view of a particle detecting system made in accordance with the invention;  
         [0025]    [0025]FIG. 4 is a graphical representation of multiple light scattering events as detected using the particle detecting system of FIGS. 2 and 3;  
         [0026]    [0026]FIG. 5 is a graphical representation of typical pulse-particle envelopes detected using the system of FIGS. 2 and 3;  
         [0027]    [0027]FIG. 6 is an enlarged graphical representation of a pulse-particle envelope using the processing method of the present invention;  
         [0028]    [0028]FIG. 7 is a graphical representation of filtered and non-filtered noise pulses in the time domain;  
         [0029]    [0029]FIG. 8 is a typical pulse envelope and forward and reverse autocorrelation so as to identify the start and end of the pulse particle envelope;  
         [0030]    [0030]FIG. 9 is a typical pulse particle envelope and an associated Gaussian fit adjusted for mean, width, and amplitude adjustments, respectively thereof;  
         [0031]    [0031]FIG. 10 is a graphical representation of a series of scattering pulse amplitude distributions; and  
         [0032]    [0032]FIG. 11 is a flowchart indicating the method in accordance with an embodiment according to the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0033]    Referring to FIG. 1, there is shown an ISPM (in situ Particle Measurement) system in accordance with the prior art. The system  10  includes a laser diode  14  which emits a fixed collimated light beam  24  across a measurement volume, such as a pump line  22 . A beam dump  18  disposed oppositely from the laser diode  14  is used to collect the resulting light beam  24  in order to minimize background light. Particles  37  crossing the measurement volume cause the light beam  24  to back, forward or side scatter with the resulting light  31  being detected by an orthogonally arranged detector assembly  30  having a photocell  34  which receives focused scattered light through an collection optical system  32 . Peak detection is employed on resulting voltage spikes from the scattered fixed beam to count particles over a preset threshold. As note, the light beam of the above ISPM system is fixed; therefore, the light beam can only impinge upon a particle moving through the measurement volume a single time.  
         [0034]    Referring to FIG. 2, there is pictorially shown an ISPM particle sensor assembly  40  in accordance with the present invention which can be used in conjunction with a semiconductor processing apparatus. The above assembly  40  includes a transmitter  44  and a receiver  48 .  
         [0035]    The transmitter  44  includes a laser diode  52  as well as a scanning mirror  56  which is oscillated by a motor (not shown) wherein a resulting scanning laser beam  60  exits into a measurement volume, such as a pump line  66 . The laser beam  60  sweeps the pump line  66  at a predetermined frequency.  
         [0036]    The receiver  48  is arranged oppositely from the transmitter  44  and is arranged to receive the scanned laser beam  60 . The receiver  48  includes a beam stop  70  that collects direct laser light. Particles  78  within the measurement volume scatter light from the scanned laser beam  60 , shown as  80 , which is directed through an set of collecting optics  74  to a photo-multiplier tube  68 . As opposed to the previously illustrated detection system, particles can be impinged many times by the scanned laser beam  60 , depending upon the particle&#39;s size and velocity.  
         [0037]    [0037]FIG. 3 illustrates a more detailed version of the pictorial particle detector assembly of FIG. 2 in accordance with a preferred embodiment of the invention. The detector assembly  100  includes three interconnected major sections comprising a transmitter  104 , a receiver  108  and a spool section  112  which interface with a processing tool (not shown). The spool section  112  is a cylindrical hollow pipe-like portion which includes flanges  116 ,  118  permitting the section to be attached to respective portions of the pump line of the processing tool. As shown herein, particles exiting from the tool&#39;s processing chamber (not shown) enter the pump line (not shown) and are directed through the spool section  112  as depicted by arrow  120 .  
         [0038]    The transmitter  104  includes a laser  124 , such as a laser diode, which is arranged within a housing  128  as well as a scanning element  132 , such as a rotating mirror, which is driven by a motor (not shown). Other optical means can be provided in lieu of a rotating mirror, such as a resonant scanner or other known means in order to create a scanned beam having a desired shape and orientation. According to this embodiment, the resulting laser beam  136  is scanned at 8 kHz, though it should be readily apparent that the scanner frequency and period of scan can easily be varied. The scanned laser beam  136  exits the transmitter  104  through a window  140  provided in the spool section  112 . Details relating to the working and operation of the scanning element  132  are known and do not in and of itself form an essential part of the invention except where otherwise indicated herein. Additional mirrors (not shown) are positioned within the housing  128  which direct the scanned beam to the window  140  with stray light being reduced by means of a plurality of slits  129 .  
         [0039]    The receiver  108  includes a beam stop or dump  148  which is disposed along an optical path arranged diametrically opposite from the transmitter window  140 . The receiver  108  also is defined by a housing  156  which includes a beam dump  148  and an adjacent window  152  which collects only scattered light from the laser beam  136  and directs same through a set of collection optics  160 , including a spectral filter  161 , toward a highly sensitive photo-multiplier tube  168  that is disposed adjacent to a proximal end of the receiver housing along with a suitable power supply  170 . The light which is scattered from particles that are present in the measurement volume is therefore directed upwardly (according to FIG. 3) toward the photo-multiplier tube  168  while the majority of the remaining light not scattered by particles moving in the measurement volume is collected by the beam dump  148 . Connectors  174 ,  176  on the proximal end of the receiver housing  156  and the transmitter housing  128 , respectively, accommodate an inline signal processor  178  having a discriminator, peak detector and microprocessor having memory and appropriate software for performing postprocessing as described below. It should be noted that in lieu of the attached signal processor, alternately the pulse data, once discriminated, can alternately be stored onto a computer disk through a suitable interface (not shown) for offline post signal processing.  
         [0040]    Since the scan velocity of the laser beam is considered high compared to the typical speed of a particle passing through the measurement volume, a typical particle will cause several pulses of scattered light while in the measurement volume. According to the present embodiment, the scan period of the laser is about  250  microseconds at 4 kHz.  
         [0041]    A typical representation is shown in FIG. 4, which depicts the various events that can occur with time being represented on the x-axis and photomultiplier voltage being expressed along the y-axis. Those events which can occur include electronic background noise, as caused by a myriad of sources such as vibration, optical noise (RF plasma) and optical interference due to the windows, among others, and particle scattering. Particles vary with speed and size, and therefore produce different light scattering pulse envelopes, such as  200  and  202 . For example, it is difficult to discern between among the envelope of a medium sized high speed particle  202  and certain forms of noise, represented as  207 .  
         [0042]    To better scrutinize and isolate these events and to better eliminate false counts, a scattered light pulse train was empirically derived from an actual measurement using a sensor instrument such as that shown in FIG. 3, which is installed on a semi-conductor processing tool. Under these tool conditions, the worst case standard deviation of the background noise level was 50 mV, and the discrimination level was set to three times the background noise level (i.e. at 150 mV). The frequency of all of the scattering pulses rising above the discrimination level (defined as) empirically from data collected early in the process tool&#39;s maintenance cycle and near the close of the maintenance cycle. Based upon this data, the range off f 3σ  was found to lie between: 
         0.19 Hz&lt; f   3σ &lt;1.86 Hz 
         [0043]    with an average f 3σ  of 0.86 Hz. Note that this false count rate also includes valid particle signals that will also survive the signal-processing algorithm. Consequently, this calculated false count rate is a worst case assessment of the impact from background noise levels.  
         [0044]    The distributions of scattering pulses for four separate production runs or lots are shown in FIG. 10. Increasing time is represented on the x-axis while increasing pulse amplitude is shown on the y-axis. These scattering amplitude distributions include both signal and noise pulses. However, it is observed that the “noise” pulses will dominate the shape of the distribution, since there are ˜100 or more noise pulses that occur for each signal pulse.  
         [0045]    As discussed previously, the present signal-processing algorithm accepts any pulse which correlates to another pulse within 250±10 μs which as noted above is the period of the scanning mirror. Calculating the area under the Poisson curve between 245 μs and 255 μs (and normalizing it with the total area under the curve) yields the probability of two noise pulses correlating together at the scanner frequency (P c ), given as:  
         P   c     =         ∫     245      µs       255      µs                F   p          (   s   )               s             ∫   0   ∞              F   p          (   s   )               s                                 
 
         [0046]    where “F p (s)” is the Poisson distribution function and “s” is the spacing of the scattering pulses. The probability of two noise pulses occurring together (based upon the scattering amplitude distributions shown in Figure) is summarized for four different product lots in Table 1.  
                                           TABLE 1                           Probability of Two Noise Pulses                Lot #   Probability (P c )                            2   0.016%           4   0.045%           5   0.090%           9   0.039%           Average   0.048%                      
 
         [0047]    The frequency of correlated noise pulses over the discrimination level (which is set at three times the background noise level), f 3σ , is given by: 
           f   c3σ   =f   3σ ×( Pc ) n−1   
         [0048]    where “f 3σ ” is the frequency of correlated noise pulses rising above the discrimination level, and “n” is the number of correlated pulses within the pulse train. The frequency of correlated noise pulses, and the number of false counts that will occur within a product lot (of 25 wafers), is tabulated in Table 2 for the cases of two-, three-, and four-correlated pulses.  
                                 TABLE 2                           Prediction of False Count Rates                # Correlated   Frequency (fc3σ)   Expected Number of           Pulses   (Hz)   False Counts Per Lot*                       2 pulses   4.1 × 10 −4     2.9           3 pulses   2.0 × 10 −7     1.3 × 10 −3             4 pulses   9.5 × 10 −11     6.3 × 10 −7                                    
 
         [0049]    This analysis indicates that the maximum false count rate is less than three false counts per lot (˜0.1 false counts per wafer), when the discrimination level is set at three times the standard deviation of the background noise level (e.g., at a 3:1 signal-to-noise level). If the discrimination level is raised even higher (from 150 mV to 300 mV), the resulting false alarm rate is predicted to decrease by over two orders of magnitude. Alternately, the false alarm rate can be improved by nearly three orders of magnitude, by requiring at least three correlated pulses to be detected (versus two correlated pulses) for a particle count to be discriminated from the background noise levels.  
         [0050]    It has been further demonstrated that a discriminator setting of four times the standard deviation of the noise will provide sufficient protection against false alarms. This can be more easily accomplished by setting the discriminator with respect to the highest observed standard deviation for the entire process (typically this is the chamber clean) at the expense of size sensitivity. However, to maintain maximum sensitivity over the entire process, it will be necessary to periodically auto-set the discriminator. This can be easily accomplished by temporarily reducing the discriminator, taking a sample of the noise, calculating the standard deviation and setting the discriminator to four times the measured standard deviation. This technique will account for all changes in noise, whatever the cause (RF, dirty windows, electronics problems, etc).  
         [0051]    A method according to the invention is described according to the flowchart shown in FIG. 11. According to the method, the light pulses that are scattered by each scanned particle crossing through the measurement volume are initially detected by a photo-multiplier  168 , step  236 . The electrical current output from the photo-multiplier  168  is then converted by conventional means into a voltage pulse, which is subsequently amplified such as for example, by means of a high-speed, two stage analog amplifier. A level-triggering device, such as a discriminator, step  240 , eliminates those signals that do not exceed a minimum threshold value, step  238 .  
         [0052]    The affect of this algorithm is illustrated in FIG. 7. The upper data window of this figure illustrates a discriminated pulse stream, in which noise pulses failing to reach a predetermined threshold value (e.g. 100 mV) having been filtered from consideration. The lower window of this figure depicts the data as it has been filtered in the time domain. The only data that remains in the lower window is that replicated at the 4 kHz scan rate.  
         [0053]    The pulse train exiting the discriminator is analyzed with a peak-detector, each contained within the processor  178  to determine the maximum value of the pulse, step  244 . Details for performing this operation are well known in the field and require no further discussion herein. This value, as well as the pulse time stamp (micro-second resolution) is stored, step  246 , for example, on a computer hard disk or in the memory of a microprocessor of the processor  178 , for post processing using a preferred signal-processing technique which is described below.  
         [0054]    The present method determines whether the pulse is representative of a particle in the scanned measurement volume by determining whether or not the pulse is part of a PPE (a pulse-particle envelope) and generating the envelope if one does exist. This determination is accomplished by looking either ahead and/or behind a representative first pulse which has been time-stamped and then utilizing the microprocessor to evaluate the scattered light distribution to locate a second pulse that lags and/or leads the first pulse by substantially the scanner period for a particular time period window.  
         [0055]    After the removal of noise pulses, the signal-processing algorithm begins searching for PPE&#39;s. According to the present method, FIG. 11, an autocorrelation calculation is performed in the forward and reverse directions (again in the time domain), step  248 , in order to identify the beginning and end of an individual PPE using the microprocessor. For purposes of discernment for the embodiment shown in FIG. 3, the 4 kHz scanner frequency is selected having a 250 μs period (+/−10 μs) in order to perform the autocorrelation. As shown in FIG. 8, and at the onset of an exemplary PPE, the autocorrelation is high in the forward direction and zero in the reverse autocorrelation. Conversely and at the end of the envelope, autocorrelation is high in the reverse direction and zero in the forward direction. The autocorrelation calculation must be performed over a time period that avoids inclusion of subsequent particle scattering envelopes. In this particular instance, a two milliseconds (0.002 sec) time period was selected. The selection of an appropriate period is somewhat arbitrary and can be tailored to particle concentration and data rate.  
         [0056]    Through this approach, the beginning and end of the particle-pulse envelope (PPE) can be identified. A set of typical PPEs are illustrated in FIG. 5. As shown in this Fig, these envelopes can take on different forms, depending on the speed (e.g., velocity) of the particle as well as the size thereof. As shown, an envelope for a relatively small and slow moving particle is shown as  220 , with a medium sized and faster moving particle having a much narrower profile  224  (envelope) and a particle having a larger size (diameter) and moderate speed such as  228  being shown for contrast.  
         [0057]    Referring to FIGS. 9 and 11, after the particle-pulse envelope has been identified as in the preceding auto-correlation calculation, step  254 , a Gaussian fitting routine is applied. Specific details relating to performing this type of statistical fit are known in the field. The algorithm begins with an assumed Gaussian mean, width, and amplitude. Each of these values are subsequently iterated to produce minimum error between the peak pulse amplitudes and the value of the Gaussian fit. The fitting procedure for the mean, width and amplitude is illustrated in FIG. 9. The deficiency of the above algorithm is that it only finds the local minimum error. However, if one considers the prohibitively long calculation required to determine a global minimum of three variables, and the fact that high accuracy is not required for these variables, the local minimum approach becomes more attractive. The created Gaussian fit should preserve the maximum pulse amplitude observed in the particle-pulse envelope. This is also a valid approach that could be used to suitably modify the algorithm described previously.  
         [0058]    As noted, the Gaussian fit once obtained on each of the particle-pulse envelopes, can lead to a valid determination of particle size and velocity, step  258 . Once the fit is obtained, the peak amplitude  230  of the Gaussian fit is used to determine particle size while the width  240  of the envelope is determinative of speed as shown in FIG. 6. More particularly, the slope of the Gaussian fit as coupled with the diameter of the laser beam at the sample volume is used to determine particle speed. One known technique for determining speed using a Gaussian fit is disclosed for example, in Holve, D. J. entitled: “Transit Timing Velocimetry (TTV) for Two-Phase Reacting Flows”, Combustion and Flame, 48:105 (1982), the entire contents of which are herein incorporated by reference.  
         [0059]    Referring to FIG. 11, one exception must be made to the autocorrelation algorithm described above. This exception refers to extremely fast particles which are contacted by the scanning beam a single time and therefore do not produce a pulse particle envelope (PPE). These events must still be validly counted by the above apparatus and distinguished from the noise produces by RF, optical and electronic effects and other means. According to this exception and as described above, the signal processing algorithm does not throw out uncorrelated peaks if the amplitude of the peaks is four times larger than the background noise level. The probability of the noise rising above this high, even during chamber clean, is extremely negligible and is a 12 sigma event. As shown in the flowchart and after if has been determined that the peak is uncorrelated, step  260 , a determination is made as to whether the pulse amplitude exceeds four times the background level to be distinguished as a single pulse event (a particle), step  264 . If yes, step  268 , the data is stored and the particle is validly counted, step  270 . If no, step  272 , then the data is discarded, step  276 .  
         [0060]    In closing, we further note the following supporting documents which are herein cross-referenced incorporated by reference in their entirety:  
         [0061]    1. P. G. Borden, (1990) ‘Monitoring Particles in Production Vacuum Process Equipment Part 1: The Nature of Particle Generation’, Microcontamination, 8(1): p. 21-24, 56-57.  
         [0062]    2. P. G. Borden (1990) ‘Monitoring Particles in Production Vacuum Process Equipment Part 2: Implementing a Continuous Real-Time Program’, Microcontamination, 8(2): p. 23-27, 61.  
         [0063]    3. P. G. Borden (1990) ‘Monitoring Particles in Production Vacuum Process Equipment: Data Collection, Analysis, and Subsequent Action for Process Optimization’, Microcontamination, 8(3): p. 47-51.  
         [0064]    4. P. G. Borden (1991) ‘Monitoring Vacuum Process Equipment: In Situ Monitors—Design and Specification’, Microcontamination, 9(1): p. 43-47.  
         [0065]    5. Holve, D. J. “Transit Timing Velocimetry (TTV) for Two-Phase Reacting Flows.” Combustion and Flame, 48:105 (1982).  
       PARTS LIST FOR FIGS.  1 - 11   
       [0066]    [0066] 10  system  
         [0067]    [0067] 14  diode  
         [0068]    [0068] 18  beam dump  
         [0069]    [0069] 22  pump line  
         [0070]    [0070] 24  light beam  
         [0071]    [0071] 30  detector assembly  
         [0072]    [0072] 31  scattered light  
         [0073]    [0073] 32  collection optics  
         [0074]    [0074] 34  photocell  
         [0075]    [0075] 37  particles  
         [0076]    [0076] 40  assembly  
         [0077]    [0077] 44  transmitter  
         [0078]    [0078] 48  receiver  
         [0079]    [0079] 52  laser diode  
         [0080]    [0080] 56  scanning mirror  
         [0081]    [0081] 60  laser beam  
         [0082]    [0082] 66  pump line  
         [0083]    [0083] 68  photo-multiplier tube  
         [0084]    [0084] 70  beam stop  
         [0085]    [0085] 74  collecting optics  
         [0086]    [0086] 78  particles  
         [0087]    [0087] 80  scattered light  
         [0088]    [0088] 100  detector assembly  
         [0089]    [0089] 104  transmitter  
         [0090]    [0090] 108  receiver  
         [0091]    [0091] 112  spool section  
         [0092]    [0092] 116  flange  
         [0093]    [0093] 118  flange  
         [0094]    [0094] 120  arrow  
         [0095]    [0095] 124  laser  
         [0096]    [0096] 128  housing  
         [0097]    [0097] 129  slits  
         [0098]    [0098] 132  scanning element  
         [0099]    [0099] 136  laser beam  
         [0100]    [0100] 140  window  
         [0101]    [0101] 144  measurement volume  
         [0102]    [0102] 148  beam dump  
         [0103]    [0103] 152  window  
         [0104]    [0104] 160  collection optics  
         [0105]    [0105] 168  photo-multiplier tube  
         [0106]    [0106] 170  power supply  
         [0107]    [0107] 174  connector  
         [0108]    [0108] 176  connector  
         [0109]    [0109] 178  processor  
         [0110]    [0110] 200  particle pulse envelope  
         [0111]    [0111] 202  particle pulse envelope  
         [0112]    [0112] 207  noise pulse  
         [0113]    [0113] 220  envelope  
         [0114]    [0114] 224  envelope  
         [0115]    [0115] 228  envelope  
         [0116]    [0116] 230  peak amplitude  
         [0117]    [0117] 234  envelope width  
         [0118]    [0118] 236  step  
         [0119]    [0119] 238  step  
         [0120]    [0120] 240  step  
         [0121]    [0121] 242  step  
         [0122]    [0122] 244  step  
         [0123]    [0123] 246  step  
         [0124]    [0124] 248  step  
         [0125]    [0125] 249  step  
         [0126]    [0126] 250  step  
         [0127]    [0127] 254  step  
         [0128]    [0128] 258  step  
         [0129]    [0129] 260  step  
         [0130]    [0130] 264  step  
         [0131]    [0131] 268  step  
         [0132]    [0132] 270  step  
         [0133]    [0133] 272  step  
         [0134]    [0134] 276  step  
         [0135]    While the present invention has been particularly shown and described with reference to the preferred mode as illustrated in the drawing, it will be understood by one skilled in the art that various changes in detail may be effected therein without departing from the spirit and scope of the invention as defined by the claims.