Class 1: What does it mean?

Class 1: What does it mean?

Tour guides conducting a group of visitors through their spanking new semiconductor facility, or other precision manufacturing facility, are often asked, “what cleanliness class is this area?” The answer better be Class 1 (or lower) if the aura of being the latest and greatest state-of-the-art facility in 1999 is to be maintained.

by Robert P. Donovan

“Class 1” implies world class, and nothing less will do. A tour guide may even confide, sotto voice, that the facility is usually cleaner than Class 0.1 but “officially we claim just Class 1.”

It was under such circumstances that I, as a member of a tour, witnessed two Japanese visitors exchange knowing glances and silently roll their eyes in reaction to such a pronouncement. I caught up with the visitors after the tour and mentioned noting their reaction. “No way that room is Class 1; look at those return ducts along the wall and those gaps in the ceiling filters.”

Later still, I approached the tour guide and, diplomatically of course, inquired how he could be sure of the room classification. “Why I took a particle counter into the room just last week and it counted only 10 particles in the 2 hours I left it running over there by that bench. It`s a 1 cfm counter so we`re talking 10 particles in 120 cubic feet. That`s better than Class 0.1!”

So who`s right?

Fed-Std-209 defines cleanroom classifications in terms of the concentration of aerosol particles that are 0.5 micron in diameter or larger. Class 1, according to the standard, means that aerosol particles with diameters of 0.5 micron or larger can be present in concentrations no greater than 1 particle/cubic foot. Similarly, Class 10 means that the concentration of particles of this diameter can not exceed 10 cubic feet and so on. The class name defines the maximum allowable concentration of aerosol particles, in units of cubic feet, that are 0.5 micron in diameter or larger. On the face of it, it would seem by these definitions that our tour guide is on solid ground for his claim, assuming his measurements refer to particles in the 0.5 micron and larger bin size. There is a rub, however. In spite of its simplicity and ease, taking an occasional single reading at a single location arbitrarily selected inside a cleanroom hardly qualifies as an adequate verification of the cleanliness classification of that cleanroom.

Fed-Std-209E is explicit on this point. In addition to defining the classes of cleanliness, Fed-Std-209E spells out the details of how such a classification must be verified. The number of locations that must be sampled, the minimum number of independent samples that must be taken at one location, the minimum volume of air that must be sampled in one measurement at a location are all part of verification procedures contained in Fed-Std-209E. It also includes statistical procedures for analyzing and interpreting the particle counts. Verification of cleanroom cleanliness classification is not a trivial exercise, and glib explanations based on a single data point, reflect ignorance of what a cleanroom classification means. Relying on random point measurements conducted as an added assignment by fab operating personnel, while perhaps appropriate for monitoring the status of a previously verified room classification, is not a defensible procedure for verifying that the performance of your new cleanroom meets its procurement specifications. And for sure, you won`t fool all your visitors all the time.

Robert P. Donovan is a contract process engineer assigned to Sandia National Laboratories by L & M Technologies Inc., Albuquerque, NM.


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