A study was initiated in an IC manufacturing clean-room in Allentown, Pennsylvania to evaluate ultrafine airborne particle contamination in the processing environment. Airborne concentrations of > 0.01-mu-m particles were continuously monitored at five locations within the cleanroom. The data exhibited a background concentration of typically 50-100 particles/ft3 on which were superimposed "hot spot" incidents with concentrations as high as 10(5) particles/ft3. A statistical evaluation was undertaken to find explanations for (a) the short term readings of high particle concentration and (b) longer term fluctuations in the background levels. No correlations across sampling locations were found for (a), indicating that the majority of these events were caused by very localized activities within the cleanroom. There were also no correlations between the event frequency for high particle concentrations and time of day, except that few events were evident when the cleanroom was quiet. For example, no such events were found over a 3-day holiday period in September. Regarding (b), a strong low-frequency correlation for particle concentrations was found among the five locations, indicating that a common array of sources was responsible for the background fluctuations. Comparison of the particle concentrations with outdoor environmental data available from the Pennsylvania Department of Environmental Control for the Bethlehem/Allentown area showed a convincing correlation between the soiling index (a standard measure of airborne particle concentrations) and background particle concentrations inside the cleanroom.
