1、 Institute of Environmental Sciences and Technology IEST-RP-CC042.1 Contamination Control Division Recommended Practice 042.1 Sizing and Counting of Submicrometer Liquid-borne Particles Using Optical Discrete-particle Counters Arlington Place One 2340 S. Arlington Heights Road, Suite 100 Arlington H
2、eights, IL 60005-4516 Phone: (847) 981-0100 Fax: (847) 981-4130 E-mail: informationiest.org Web: www.iest.org 2 IEST 2011 All rights reserved Institute of Environmental Sciences and Technology IEST-RP-CC042.1 IEST-RP-CC042.1 Institute of Environmental Sciences and Technology IEST 2011 All rights res
3、erved 3 This Recommended Practice is published by the Institute of Environmental Sciences and Technology to advance the technical and engineering sciences. Use of this document is entirely voluntary, and determination of its applicability and suitability for any particular use is solely the responsi
4、bility of the user. Use of this Recommended Practice does not imply any warranty or endorsement by IEST. This Recommended Practice was prepared by and is under the jurisdiction of Working Group 042 of the IEST Con-tamination Control Division. Copyright 2011 by the Institute of Environmental Sciences
5、 and Technology First printing, October 2011 ISBN 978-0-915414-09-3 PROPOSAL FOR IMPROVEMENT: The Working Groups of the Institute of Environmental Sciences and Technology are continually working on improvements to their Recommended Practices and Reference Docu-ments. Suggestions from users of these
6、documents are welcome. If you have a suggestion regarding this doc u-ment, please use the online Proposal for Improvement form found on the IEST website at www.iest.org. Institute of Environmental Sciences and Technology Arlington Place One 2340 S. Arlington Heights Road, Suite 100 Arlington Heights
7、, IL 60005-4516 Phone: (847) 981-0100 Fax: (847) 981-4130 E-mail: informationiest.org Web: www.iest.org 4 IEST 2011 All rights reserved Institute of Environmental Sciences and Technology IEST-RP-CC042.1 IEST-RP-CC042.1 Institute of Environmental Sciences and Technology IEST 2011 All rights reserved
8、5 Sizing and Counting of Submicrometer Liquid-borne Particles Using Optical Discrete-particle Counters IEST-RP-CC042.1 CONTENTS SECTION 1 SCOPE AND LIMITATIONS . 7 2 REFERENCES 7 3 TERMS AND DEFINITIONS . 8 4 BACKGROUND AND PURPOSE . 9 5 LIQUID-BORNE PARTICLE MEASUREMENTCHALLENGES AND RECOMMENDED PR
9、ACTICES . 9 FIGURE 1 SCHEMATICS OF THE OPTICAL SYSTEM OF A LIQUID-BORNE PARTICLE COUNTER BASED ON LIGHT SCATTERING 10 2 TYPICAL POWER LAW DISTRIBUTIONS FOUND IN MANY LPC APPLICATIONS. 13 3 STATISTICS OF PSL STANDARDS . 14 4 50/50 SPLIT CALIBRATION OF AN LPC 14 5 50/50 SPLIT CALIBRATION OF AN LPC (AL
10、L CHANNELS CALIBRATED THE SAME WAY) . 14 6 COUNTING EFFICIENCY OF REAL-WORLD PARTICLES. 15 7 PARTICLE SIZE DISTRIBUTION AT DIFFERENT CONCENTRATION LEVELSEFFECT OF COINCIDENCE ERROR 16 8 A TYPICAL PULSE HEIGHT DISTRIBUTION FOR A SPECTROMETER. 17 9 EFFECT OF EXCESSIVE BUBBLES ON THE SIZE DISTRIBUTION
11、OF PARTICLES 18 10 SCATTERED LIGHT INTENSITY BY A PARTICLE (0.2 m AT 633 nm WAVELENGTH) AS A FUNCTION OF REFRACTIVE INDEX CONTRAST 20 11 EXCESSIVE BASELINE NOISE EFFECT 22 12 PROPER INSTALLATION OF AN LPC TO MONITOR A PARTS-CLEANING ULTRASONIC BATH . 23 13 PROBABILITY DENSITY FUNCTION FOR 10-MIN SAM
12、PLE TIME . 26 14 PROBABILITY DENSITY FUNCTION FOR 30-MIN SAMPLE TIME . 27 6 IEST 2011 All rights reserved Institute of Environmental Sciences and Technology IEST-RP-CC042.1 TABLE 1 ADVANTAGES AND DISADVANTAGES OF BATCH SAMPLING INSTRUMENT TYPES . 11 2 DATA FORMAT FOR AN LPC ILLUSTRATING DIFFERENTIAL
13、 AND CUMULATIVE COUNTS . 12 3 INDEX CONTRAST OF CERTAIN PARTICLE-FLUID COMBINATIONS . 21 4 SENSOR-TO-SENSOR COUNT CORRELATION (AT CONCENTRATION LEVELS WITH LOW COINCIDENCE ERRORS) . 24 5 STANDARD DEVIATIONS OF LOW-CONCENTRATION SCENARIOS BASED ON POISSON DISTRIBUTION 25 6 PROBABILITY THAT A SAMPLE W
14、ILL CONTAIN A GIVEN NUMBER OF PARTICLES . 26 7 CUMULATIVE PROBABILITIES OF AN ALARM BEING FALSE AS A FUNCTION OF AVERAGE PARTICLES PER SAMPLE . 28 APPENDIX A EXAMPLE: EVALUATING THE LIKELY NUMBER OF FALSE ALARMS PER DAY FOR A GIVEN SAMPLE TIME 29 B BIBLIOGRAPHY . 30 IEST-RP-CC042.1 Institute of Envi
15、ronmental Sciences and Technology IEST 2011 All rights reserved 7 Institute of Environmental Sciences and Technology Contamination Control Division Recommended Practice 042.1 Sizing and Counting of Submicrometer Liquid-borne Particles Using Optical Discrete-particle Counters IEST-RP-CC042.1 1 SCOPE
16、AND LIMITATIONS 1.1 Scope This Recommended Practice (RP) addresses the sizing and counting of submicrometer liquid-borne particles using optical discrete-particle counters with a focus on applications in the semiconductor, flat-panel display, and data storage industries. Top-ics covered include the
17、following: Overview of light-scattering technology Liquid particle counter (LPC) instrument types Data interpretation Coincidence level or maximum concentra-tion limit Particle-size detection limit Bubble issue Refractive index effect Flow rate Calibration verification Instrument noise and false cou
18、nts Practice of minimizing sensor contamination Sensor correlation Particle counting statistics 1.2 Limitations This RP does not address the measurement and identi-fication of living organisms, e.g., bacteria. This RP does not address the measurement of extreme-ly high particle concentrations, e.g.,
19、 chemical-mechanical polishing (CMP) slurry. This RP does not address the calibration of LPCs. 2 REFERENCES The cited editions of the following documents are incorporated into this Recommended Practice to the extent specified herein. Users are encouraged to in-vestigate the possibility of applying t
20、he most recent editions of the references. 2.1 Reference Documents IEST-RD-CC011.2: A Glossary of Terms and Defini-tions Related to Contamination Control ISO 21501-2:2007 Determination of particle size distributionSingle particle light interaction me-thodsPart 2: Light scattering liquid-borne partic
21、le counter 2.2 Sources and Addresses IEST Institute of Environmental Sciences and Technology 2340 S. Arlington Heights Road, Suite 100 Arlington Heights, IL 60005-4516, USA Phone: 847-981-0100 Fax: 847-981-4130 www.iest.org ISO International Organization for Standardization 1, ch. de la Voie-Creuse,
22、 Case postale 56 CH-1211 Geneva 20, Switzerland Phone: +41 22 749 01 11 Fax: +41 22 733 34 30 www.iso.ch 8 IEST 2011 All rights reserved Institute of Environmental Sciences and Technology IEST-RP-CC042.1 3 TERMS AND DEFINITIONS The following terms have special meaning in the con-text of this RP. See
23、 also IEST-RD-CC011. capillary (also flow cell) The conduit through which sample fluid passes through the illuminating path. NOTE: The entire flow path may not be illuminated. See LPC, volumetric, and LPC, non-volumetric. channel (also size bin) A bin for recording counts of particles within a speci
24、-fied size range. Channels are established by means of voltage thresholds that match particle light-scattering intensity to particle size. coincidence The simultaneous presence of two or more particles in the sensing volume of the instrument, causing the instrument to interpret the combined signal e
25、rro-neously as resulting from one larger particle. coincidence level (also maximum particle number concentration) The particle concentration specified by the manu-facturer of the LPC at which the error due to coin-cidence is 10% or less. (Manufacturers may specify concentration limits at error level
26、s other than 10%.) counter A type of LPC typically having several channels (see section 5.2.1). This term is also used generical-ly to refer to particle measurement instruments, as in “liquid-borne particle counter.” counting efficiency The ratio of the measured result of a light-scattering liquid-b
27、orne particle counter to the measured result of a reference instrument using the same sample (see ISO 21501-2: 2007). false count rate (also false count, noise, noise rate, zero count rate) The maximum particle count indicated by a particle counter, in a given volume, that is sampling par-ticle-free
28、 liquid. This value is specified by the LPC manufacturer. flow rate, effective (also actual measured sample flow rate) The flow rate of the introduced sample that is illu-minated within the viewing volume region. Only particles passing through this region will be de-tected. Effective flow rate is us
29、ed to determine par-ticle concentration. flow rate, sampling The volumetric rate at which a sample liquid passes through the particle sensor. Typically, particle sen-sors are calibrated at specific sampling flow rates. For liquid particle counters, units are typically ex-pressed in milliliters per m
30、inute (mL/min). liquid-borne particle counter (LPC) A type of optical particle counter that uses light to count and size particles suspended in a liquid medium by detecting light scattered by discrete particles. LPC, volumetric A type of LPC in which the laser beam illuminates the entire sample volu
31、me to measure particles using a uniform intensity of laser beam shape. LPC, non-volumetric A type of LPC in which the laser beam illuminates only a portion of the sample volume to measure particles. monitor A type of LPC that typically has poor resolution and few channels but high sensitivity. optic
32、al equivalent size The equivalent size of particles, as reported by an LPC, that would scatter the same amount of light into a defined solid angle element as the spherical refer-ence particle. particle-size detection limit The smallest size at which the counting efficiency is 50%. This definition on
33、ly applies to spectrometers and counters. particle size distribution (PSD) A list of values (typically a number of counts) or a mathematical function that defines the relative num-bers of particles present, sorted according to particle sizes or size ranges. refractive index contrast The ratio of the
34、 refractive index of a particle to the refractive index of the suspending medium. sample time Typically, a selectable period during which particles are counted and sized. size resolution A measure of the ability of an instrument to distin-guish between particles of different sizes, typically express
35、ed as a percentage at a specified particle size, referring to the ratio of size variance at that size to the mean size. IEST-RP-CC042.1 Institute of Environmental Sciences and Technology IEST 2011 All rights reserved 9 spectrometer A type of LPC used to measure PSD; spectrometers provide high resolu
36、tion and many channels. volume, effective sampling The portion of the sample that has passed through the sensing zone of the particle sensor. Effective sampling volume = sampling volume viewing volume %. volume, sampling The total volume of liquid that has passed through the particle sensor constitu
37、ting a discrete sample measurement. volume, viewing (%) The portion of the sample flow path, expressed as a percentage, that is illuminated by the incident illumi-nation of the particle sensor. 4 BACKGROUND AND PURPOSE Liquid-borne particle measurement in the semicon-ductor industry involves many te
38、chnical challenges, which, if not handled properly, can substantially af-fect the measurement results in both sizing and counting of particles. Research work and technical papers have addressed the technical challenges indi-vidually. What is lacking is a single source that cov-ers a broad range of t
39、hose challenges and solutions and provides a handy reference for professionals in this field. This RP is intended to fill that gap. 5 LIQUID-BORNE PARTICLE MEASUREMENTCHALLENGES AND RECOMMENDED PRACTICES 5.1 Overview of light-scattering technology A liquid-borne particle counter (LPC) is an instru-m
40、ent that uses light to detect particles suspended in a liquid medium. Particle detection technology using light sources involves measurement by either light-scattering or light-extinction method. Light-scattering technology directly measures the amount of light scattered by particles to obtain infor
41、mation on the concentration and the size distribution of those par-ticles. LPCs employing light-scattering technology are predominantly used to detect low concentrations of particles ranging in diameter from 0.05 m to 20 m in industries such as the semiconductor industry. Light-extinction technology
42、 measures the overall loss of light intensity from the light source due to scattering by the particles collectively, and is typically used to measure high concentrations of par-ticles larger than 2 m in diameter. This RP focuses on the LPCs based on light-scattering technology. Light scattering and
43、extinction by particles are de-scribed by the Lorenz-Mie theory (Bohren right: 100% of the flow cross-section is illuminated.) since the magnitude of the voltage pulse is correlated to the amount of light scattered by a particle of a spe-cific diameter. During sampling, individual pulses are convert
44、ed into the numerical data that represent par-ticle counts. The optical response R for a single-particle light-scattering instrument is dfdddCGR s ca , where G describes the illumination and collection geometry, f includes the source spectrum and spectral sensitivity of the photodetector, and the di
45、fferential scattering cross-section ddCsca is calculated from the Lorenz-Mie theory, in which the intensity I of the light scattered by the particles depends on the incident light intensity I0, the polari-zation angle , the detection angle of the scattered light , the refractive index n, the wavelen
46、gth , and the optical size parameter : 0 , , ,I I f n . 5.2 LPC instrument types 5.2.1 Volumetric vs. non-volumetric and monitor vs. spectrometer/counter From a capability point of view, depending on the size resolution and the number of size channels, LPC instruments are categorized as spectrometer
47、, counter, or monitor. Spectrometers have the highest resolution and the greatest number of size channels. Counters may have high intrinsic optical resolution, but that resolution is reduced because fewer size channels are provided in the design of electronics to reduce cost. Monitors do not provide
48、 uniform sample volume illumination, thus they offer poor resolution. Howev-er, one of the advantages of monitors is that they al-low larger effective flow rates for the same sensitivity relative to spectrometers; hence, monitors yield more statistically reliable data in high-purity applications. Mo
49、nitors are also simpler to operate and less expensive than spectrometers and counters. From a design point of view, depending upon the effective sampling volume, LPC instruments are ca-tegorized as volumetric or non-volumetric. Volume-tric LPCs are designed to view the entire sample flow under a uniform laser beam intensity. The uniform beam shines on the particles flowing in the liquid with the same light energy, regardless of each par-ticles location in the flow path, and therefore offers high resolution. The drawback to this design is that
