Verifying a cleanroom classification

Last month`s column compared the classification systems of Fed-Std-209E and ISO 14644-1. This month`s column highlights differences in the requirements of the two standards for verifying that a given cleanroom does, in fact, meet the criteria of a given cleanroom classification.

by Robert P. Donovan

Both standards specify:

1. the type of instrumentation to be used in making measurements of particle concentration and the frequency and scope of the instrument calibration;

2. the minimum number of locations within the cleanroom at which measurements of particle concentration must be made;

3. the minimum volume of air that must be sampled during each measurement;

4. the minimum number of measurements that must be taken at each location and the minimum number of measurements that must be taken throughout the cleanroom; and

5. the statistical methods that must be followed in interpreting the measurements.

The two standards do not differ significantly in their specifications in categories 1, 3 and 5. There are also just small differences between the two in category 4. The differences in category 2 — the minimum number of locations — however, can be substantial. It is the category 2 and 4 differences that will be discussed here.

Annex B of ISO 14644-1, labeled “normative” (meaning it`s a mandatory part of the standard rather than “informative”), defines the minimum number of required sampling locations as:

NL = A 0 .5 (Eq. B.1 from Annex B of ISO 14644-1)

NL = the minimum required number of sampling locations

A = the area of the cleanroom in square meters

The minimum number of sampling locations required by Fed-Std-209E is the lesser of:

NL = A/25 or NL = A/(NC) 0 .5 (Section 5.1.3.1 of Fed-Std-209E)

A = the area of the cleanroom in square feet

NC = the numerical classification of the cleanroom (NC = 10 for a Class 10 cleanroom, 1 for a Class 1 cleanroom, etc.)

209E`s two expressions for NL are equal when NC = 625. For higher quality cleanrooms (NC < 625), the first expression (NL = A/25) yields the lesser value of NL. When NC > 625, the second expression yields the lesser value.

The minimum value of NL according to Fed-Std-209E is 2; ISO 14644-1 allows NL to take on the irreducible value of 1.

The equations show that NL varies as the square root of cleanroom area by ISO 14644-1, while in Fed-Std-209E, NL varies linearly with A. This difference becomes huge for large cleanrooms. Consider a state-of-the-art cleanroom that is 100 square meters in area. The minimum number of sampling locations required by ISO 14644-1 is 10. For this same facility, Fed-Std-209E requires 44. (When A is in square meters, the NL expression of Fed-Std-209E becomes NL = A/2.32). For larger cleanrooms the differences become much larger. When A = 1,000 square meters, the ISO NL = 32 and the Fed-Std NL = 432!

For lower quality cleanrooms, however, the differences are not as great or can even reverse in that Fed-Std-209E may require fewer sampling locations. At Class 100,000, for example, the second equation for calculating the minimum number of sampling locations required by Fed-Std-209E yields the lesser number; the 100-square-meter cleanroom considered previously requires only four sampling locations per Fed-Std-209E* rather than the 10 required by ISO 14644-1. (The minimum number of locations required by ISO does not depend on the cleanliness classification as it does in Fed-Std-209E.)

Both standards allow just one measurement to be made at any one location but specify a larger minimum number of measurements per cleanroom or clean zone. Fed-Std-209E requires a minimum of five individual measurements of particle concentration per cleanroom or clean zone. These five measurements can be divided in any manner between the minimum required two locations — four at the first location and one at the second or three and two. ISO 14644-1 requires only three individual measurements per cleanroom or clean zone. When only one location is required, these three measurements will, of course, be at that one location. This difference between the two standards in total minimum number of individual measurements per cleanroom or clean zone is relatively minor compared to the potential differences in the minimum number of required locations per cleanroom or clean zone, as pointed out in the preceding paragraphs.

Thus, while the two standards read so much alike that they seem to be carbon copies except for the exclusively metric units of the ISO standard, there are, in fact, some very different requirements. To avoid unpleasant surprises, all parties to agreements invoking cleanroom design and performance “according to the standards” need to be aware of the differences.

* 100 square meters is 1076 square feet so that NL = 1,076/(100,000) 0 .5 = 1,076/316 ª 3.4 ` 4

Robert P. Donovan is a process engineer assigned to Sandia National Laboratories as a contractor by L & M Technologies Inc., Albuquerque, NM. His Sandia work is developing technology for recycling spent rinse waters from semiconductor wet benches.