Indecision 2000: Particle counter version

04/01/2001

by Robert P. Donovan

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Remember the indecision in Florida associated with the 2000 presidential election? Each side insisted upon new counts, partial recounts or no recounts; new counting rules or the unconstitutionality of changing rules. It reminded me of using one particle counter, then another and perhaps still others to measure and remeasure the particle concentrations that will be used to certify the classification of a cleanroom.

Here again the hope of the parties is to eventually find a number they like, whether to support or deny the claimed classification. Like counting votes, this approach is used because folklore has it that no two particle counters ever agree with each other, even when using identical models of a particle counter made by the same manufacturer. Having a plurality of particle counters available for measuring and remeasuring particle concentrations would thus seem to increase the chances of eventually documenting a result favorable to the most persistent party.

Sorry, crafty cleanroom sellers and buyers, this dubious practice is no longer viable, if it ever was. The key is up-to-date particle counter calibration. Calibration methods can be based on either a size calibration or a count calibration.

Size is the simpler variable to control. Polystyrene latex (PSL) spheres, and PSL spheres cross-linked with other polymers, are commercially available over a broad range of diameters—from 20 nm to >100 µm. Better yet, they are available in near monodisperse size distributions—relative standard deviations are almost always less than 5 percent and typically less than 2 percent.

One merely has to order the desired size or sizes from the PSL sphere manufacturer's catalog to obtain accurate and precise PSL sphere size standards with which to begin a calibration. Typically, manufacturers ship aqueous solutions of PSL particles in small, dropper-tipped vials containing some modest concentration of surfactant to minimize agglomeration of the many small, single particles suspended in the liquid.

As shipped, the PSL particle concentrations in the vials are high—on the order of 104-1015 particles per ml, depending on diameter. The manufacturer estimates this concentration by weighing the mass of PSL particles introduced into a known volume of liquid and then converting that mass into an equivalent number of spheres having the density of PSL and the microscopically determined diameter certified from NIST-traceable values. These highly concentrated, as-shipped, solutions must be diluted, say one drop of concentrated PSL solution per 100 ml of water or more, and then aerosolized in a gas stream of known flow rate to create a suitable size calibration standard.

Once having aerosolized the PSL standard at least two size calibration procedures exist: 1.) Plot the particle count rate versus detector threshold voltage; assign the PSL diameter of the standard to that value of threshold voltage yielding the maximum differential count rate, often determined using a multichannel analyzer. This procedure calibrates the counter with respect to the mode of the PSL particle distribution. 2.) Feed the aerosol to both the counter being tested and a broadband, referee counter such as a condensation particle counter (CPC); adjust the count rate of the test counter to be 50 percent that of the referee counter.

A multichannel analyzer can also be used when the signal/noise is sufficiently high. This procedure calibrates the counter with respect to the median size of the PSL particle distribution. Size differences between the mode and the median are small for monodisperse PSL.

Count calibration also uses a referee counter but is best done with a polydisperse challenge aerosol of relatively high concentration such as found in an ISO Class 6 or 7 (Fed-Std-209E Class 1000 or 10000) cleanroom. One simply adjusts the test counter's threshold voltages to yield the same count rate as that of the referee counter for selected size cuts.

ASTM has published a standard for size calibration¹ and for count calibration.² IEST guidelines³ describe a size calibration procedure.

Dahneke and Johnson⁴ were among the first to document the improved agreement among diverse particle counters observed when freshly calibrated. Their data compare the responses of a group of four particle counters before and after size calibration. The wide variations in particle concentrations reported among the counters prior to calibration (as high as a factor of 6) were dramatically reduced after calibration (generally to within 10 percent of each other). While 10 percent may not be adequate to settle close presidential elections, it is usually tight enough to avoid classification uncertainties and disputes. Thus, up-to-date calibrations have become an essential requirement for specifying cleanroom classifications, and most manufacturers of particle counters recommend recalibration at least annually, as does RP-14.

In addition, ISO 14644-1 requires that the reports that certify cleanroom Class compliance include copies of the latest calibration tests conducted on all particle counters used in the certification procedure. The wiggle room allowing crafty certification practices has been significantly reduced.

Robert P. Donovan is a process engineer assigned to the Sandia National Laboratories as a contract employee by L & M Technologies Inc., Albuquerque, NM. His Sandia project work is developing technology for recycling spent rinse waters from semiconductor wet benches.

References

1. ASTM F328-98 Standard Practice for Calibration of an Airborne Particle Counter Using Monodisperse Spherical Particles (ASTM, 100 Barr Harbor Drive, Conshohocken, PA 19428-2959).
2. ASTM F649-80 (1999) Standard Practice for Secondary Calibration of Airborne Particle Counter Using Comparison Procedures (ASTM, 100 Barr Harbor Drive, Conshohocken, PA 19428-2959).
3. IEST CC-RP-0014-Calibration and Characterization of Optical Airborne Particle Counters (IEST, 940 East Northwest Highway, Mt. Prospect, IL 60056).
4. Dahneke, B. and B. Johnson, "A Method for Cross Calibrating Particle Counters," J. Environmental Sciences 29 (5), Sept/Oct 1986, pp 31-36.

Acknowledgement: I thank Dr. Charles Montague for his careful review of the draft of this column. The final version printed here has incorporated many of his suggestions.