Cleanrooms are isolated environmental areas where airborne contaminants including dust and bacteria are limited and controlled. Such rooms are often necessary for microelectronics and optical processes that would otherwise sustain unacceptably high rates of process degradation, yield reduction or process failure due to the presence of such microscopic contaminants.
The air quality in cleanrooms is described in Federal Standard 209(d) "Airborne Particulate Cleanliness Classes in Cleanrooms and Clean Zones" by listing the allowable number of particles of contaminant .5 micrometers or larger in a cubic foot of air. Federal Standard 209(e) overlaid metric sizes and classes, and then ISO Standard 14644 subsequently superseded and replaced Federal Standard 209, but the verbal classification of room cleanliness based upon earlier Federal Standards classification persists. The following table shows general relationships between the related class systems.
The table conversion shows that while under FS 209, a Class 100 room for example, was allowed 100 particles >.5µm, corresponding ISO Class is based on cubic meters and not cubic feet. The allowable particle count is 3,520 at .5 and 10,200 at .3µm.
CLEANROOM BASICS: PARTICLE COUNTS AND CLASSIFICATIONS