Light scattering instruments for measuring particles: The options
06/01/2000
by Robert P. Donovan
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Light scattering is the basis of the most commonly used instruments for monitoring and classifying cleanrooms as well as measuring particle emission rates from tools, filters and processes in a cleanroom.
It's a given that anyone in semiconductor or precision product manufacturing owns at least one. However, light scattering instruments go by a number of different generic names — monitors, counters, spectrometers, volumetric, in situ, aerosol, liquid-borne — so the request "get me an optical, particle-measuring instrument" doesn't settle the acquisition issue. Let me review the nomenclature and properties differentiating types of light-scattering instruments for measuring particle properties.1
In air, there are two types of optical-based instruments for counting and sizing aerosol particles: counters and spectrometers. The major differences between these two are particle size resolution and volume sampling rate. A counter classifies particles into relatively broad size ranges, say 4 to 8 size bins over an order of magnitude in size range, and does so at a volume sampling rate of up to 1cfm (28 liters per minute).
A spectrometer divides this same size range into 24, 32, 64 or even more size bins, taking full advantage of the 6th power dependence of light scattering intensity upon particle diameter that is characteristic of Rayleigh light scattering from small particles.2
However, a spectrometer samples the air at a much lower volume flow rate than a monitor, say 0.01 cfm or less vs. the 1-cfm typical of high flow rate monitors. For sampling the aerosol particle concentration in a cleanroom, using a high flow rate counter rather than a low flow rate spectrometer means that less time will be needed to sample the volume of air that is required for valid sampling by either ISO 14644-1 or Fed-Std-209E.
Assuming that the number of background counts registered per unit time by a 1-cfm particle counter is the same as that of a 0.01 cfm spectrometer implies that the high flow rate counter can better measure lower concentrations of aerosol particles because the signal-to-noise ratio of the counter will be greater than that of a spectrometer measuring the same particle concentration.
Coincidence errors cause a high flow rate counter to lose counting accuracy at lower concentrations of aerosol particles than a spectrometer. Coincidence errors arise when the scattered light signal from a particle reaches the detector before the detector has recovered from the signal generated by the preceding particle. For particle counters, coincidence errors typically exceed 10 percent when concentrations exceed 104-105/cm3. Spectrometers generally handle concentrations two or more orders of magnitude higher before exhibiting similar coincidence errors.
Thus, for measuring aerosol concentrations in a cleanroom, a particle counter is clearly the preferred choice. For assessing particle emissions and obtaining high-resolution particle size distributions emitted from a specific, prolific particle generator, a spectrometer will often be the better choice. For many intermediate concentrations of aerosol particles both counters and spectrometers can often generate usable particle data.
In measuring particles in liquids, the selection of an optical-based instrument becomes more complicated. A big part of the problem is the lack of fluid-focussing capability in liquids. A gas stream can be aerodynamically shaped so that the sample drawn into the particle counter becomes a fluid stream of desired, controlled cross-sectional area and shape.
In liquids, this stream-shaping capability is lost; the duct walls define the boundaries of the fluid stream being sampled. Unfortunately, these wall boundaries themselves scatter significant light intensity to the detector which is not related to the particle concentration in the fluid being measured. Next month's column will outline two approaches for dealing with this complication.
Robert P. Donovan is a process engineer assigned to the Sandia National Laboratories as a contract employee by L & M Technologies Inc., Albuquerque, NM.
References
- Knollenberg, R. G. and D. L. Veal, "Optical Particle Monitors, Counters and Spectrometers: Performance Characterization, Comparison and Use", 1992 SPWCC Proceedings, pp 197-240; 1991 Proceedings of the Institute of Environmental Sciences, pp 751-771.
- Hinds, W. C., Aerosol Technology, pp. 324-329, John Wiley & Sons, New York, NY, 1983.