Meeting new aerosol particle measurment challenges
11/01/2002
by Robert P. Donovan
Last month's column described the DB/CPC combination that can be used to measure aerosol particle size distributions over size ranges too small to be counted by an OPC. This class of measurement and control of aerosol particles in the 0.01- to 0.1-micron size range is becoming more important in cleanrooms as critical dimensions of silicon chips continue to decrease.
Even the most sensitive of today's optical particle counters (OPCs) detect no particles smaller than 0.05 micron, and that may be stretching it. Thus, other measuring equipment will be required to characterize cumulative particle concentrations greater than 0.01 micron.
The growing threat posed by smaller particles may lead to modified standards that do so in the future. The diffusion battery/condensation particle counter (DB/CPC) represents one combination already available for measuring the cumulative concentration of aerosol particles down to 0.01 micron, should such data be necessary.
As discussed in last month's column, the DB uses the sensitive dependence of particle diffusion coefficients upon particle size to separate aerosol particles according to size. Particles diffuse to adjacent surfaces of the DB, stick and are removed from the aerosol stream. The reduced particle concentration in the aerosol stream then becomes a measure of the particle concentration in a known range of particle diffusion coefficients and, hence, particle sizes.
Loss of small aerosol particles occurs in flows through any tube or conduit so that delivering a valid particle sample to the DB/CPC requires that the sampling lines be as short as possible and that the particle counter operate at as high a volume flow rate as possible. Even so, sampling line losses may often be significant and must be accounted for in reporting the concentrations of particles in the 0.01- to 0.1-micron size range.
In contrast, sampling line losses for aerosol particles attributable to diffusion (or, in fact, any other mechanism) in the size range 0.01 to 1.0 micron are almost always negligible. It's difficult to incorrectly sample aerosol particles in this size range; any arrangement delivers a largely unbiased sample.
Thus, contemporary cleanroom standards, based on cumulative particle concentrations greater than 0.1 or 0.3 micron in diameter, do not need to specify sample line configurations in any great detail.
The figure illustrates the type of data that can be collected with a DB/CPC in the 0.01- to 0.1-micron size range. All data plotted at particle diameters less than 0.2 micron was collected with the DB/CPC combination. Data points for particles with diameters of 0.3 micron and larger were collected by a standard OPC, which was concurrently sampling the cleanroom aerosol.
The figure also shows typical differences in aerosol particle size distributions between a cleanroom at-rest and an operational cleanroom. At-rest implies no production underway and no operating personnel in the cleanroom, although cleanroom apparatus normally left "on" (such as furnaces) is expected to be powered. Operational means normal cleanroom processes are underway with typical cleanroom personnel present and performing normal functions.
The cleanroom at-rest curve displays the aerosol particle size distribution expected downstream of a HEPA/ULPA filter. The concentration of aerosol particles smaller than ~ 0.1 micron that penetrate the filter is low because the diffusion mechanism causes such particles to be efficiently captured by the filter fibers.
Thus, the cumulative concentration curve flattens out, confirming that the aerosol particles in this cleanroom originate primarily from particle penetration through the ceiling filters. In addition, the major increase in particle concentration occurs in the 0.2- to 0.3-micron particle size range, the range of maximum particle penetration through HEPA/ULPA filters. These filters efficiently remove aerosol particles of both larger and smaller diameters.
When the cleanroom is in operation, however, the cumulative particle concentration increases and the particle size distribution changes. The presence of increased concentrations of aerosol particles in the size ranges efficiently removed by HEPA/ULPA filters (< 0.1 and > 0.3 micron) shows that these aerosol particles have been generated within the cleanroom itself—the cleanroom operations and the people running them. Internal generation of the particles dominating operational cleanrooms holds true for virtually all cleanrooms.
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Robert P. Donovan is a process engineer assigned to the Sandia National Laboratories and a monthly columnist for CleanRooms magazine. He can be reached at [email protected].