The coming debut of ISO cleanroom standards
The shift to metrication of cleanroom classifications is about to take a major step forward with the public release of ISO standards 14644-1 and -2 by the International Organization for Standardization, Geneva.
by Robert P. Donovan
These two standards define the cleanliness classifications of cleanrooms based on the concentration of particles 0.1 micron and larger in diameter per cubic meter. The new class names are defined in the main body of 14644-1. Mandatory appendices B and C (called Annexes) of 14644-1 specify the procedures for verifying a given cleanroom classification. 14644-2 specifies “testing and monitoring to prove continued compliance with ISO 14644-1.” The objectives of these international standards are identical to those of the familiar Fed-Std-209 series and indeed the lineage of these new standards can be easily traced to Fed-Std-209E. While the objectives and the general path to achieving these objectives are similar between 209 and the new ISO standards, their details differ significantly.
Gone from the new standards are Class 1, Class 10 and all the other familiar classifications of 209E. The new names are ISO Class N where 1 N 9 in increments of 0.1. As with 209, the class name tells the maximum allowed number of particles of a given diameter. In the ISO classifications, however, the maximum allowable concentration for a given class is 10N, the concentration units are particles per cubic meter, and the reference particle diameter is 0.1 micron and larger — quite different from 209E in which the Class number is itself the maximum allowable concentration of particles per cubic foot with a reference particle diameter of 0.5 micron.
How then do the 209E and the ISO standards differ in describing a given cleanroom classification? Take Class 1 of 209E, allowing a maximum particle concentration of 1 particle per cubic foot. Rounded off, this concentration corresponds to 35 particles per cubic meter but the reference particle diameter is still 0.5 micron. How does one convert that concentration at 0.5 micron into an equivalent concentration at 0.1 micron? Table 1 of Fed-Std-209E offers a ready answer: 35 particles per cubic meter at 0.5 micron correspond to 1,240 particles per cubic meter at 0.1 micron. This conversion is based on a particle size distribution approximated by an inverse power law with an exponent of 2.2 (particle concentration [particle diameter] -2.2). Multiplying 35 by (0.1/0.5) -2.2 (=1,207) doesn`t quite yield 1,240 (the numbers in Table 1 of Fed-Std-209E do not represent exact power law behavior) but, accepting a concentration of 1,240 particles per cubic meter at 0.1 micron as a reasonable approximation of 1 particle per cubic foot at 0.5 micron, the corresponding ISO Class number becomes:
1,240 = 10N
log10 (1,240) = N
N = 3.09
By this conversion, Class 1 of 209E corresponds to ISO Class 3.1 as defined by ISO 14644-1.
An alternative conversion procedure appears in ISO 14644-1. Here the power law size distribution has an exponent of 2.08. The 35 particles per cubic meter at 0.5 micron correspond to 35 (0.1/0.5) -2.08 (= 995) particles per cubic meter at 0.1 micron (or 1,000 particles per cubic meter by Table 1 of 1SO 14644-1) which rounds off to ISO Class 3.0.
Comparing Tables 1 from both standards shows that a simple relationship exists between the names used in the two classifying systems. The number appearing in the ISO Class name converts to the number appearing in the 209E Class name by simply moving the decimal point to either side of 1.0 the number of places equal to 3 minus the number appearing in the ISO Class name. For example,
ISO Class 1 Class 0.01 (1 minus 3 = -2; move the decimal point 2 places to the left)
ISO Class 2 Class 0.1
ISO Class 3 Class 1
ISO Class 4 Class 10 (4 minus 3 = 1; move the decimal place 1 place to the right)
ISO Class 5 Class 100
ISO Class 6 Class 1000, and so on.
Neither standard requires that verification be conducted at the diameter of the reference particle. However, the particle diameter or diameters at which the verifying measurements are made must be within the size range defined for each class in Table 1 of each standard. And note that the class correspondences between the two standards strictly apply only when class verification measurements are made at the 0.5 micron particle diameter; that is, the limiting concentration of Fed-Std-209E Class 1 corresponds to that of ISO Class 3 only at 0.5 micron. At 0.2 micron or 0.3 micron or other intermediate particle diameters at which class verification is allowed by either standard, the class limits will differ because of the differing power law dependencies assumed by each standard. For example, by Fed-Std-209E, 1,200 particles per cubic meter at 0.1 micron meets the concentration requirement for Class 1 but it does not meet the concentration requirement for ISO Class 3, which specifies a maximum of 1,000 particles per cubic meter. Admittedly the differences are small and may not often be important. Nonetheless, switching between the two standards will be complicated by such details. The prudent action is to switch to the ISO standard as soon as practicable.
Procedures for verification, contained in Annexes B and C of ISO 14644-1, also differ from those of Fed-Std-209E and will be reviewed next month. Some of these differences are major!
Robert P. Donovan is a process engineer assigned to the Sandia National Laboratories as a contract employee by L & M Technologies Inc., Albuquerque, NM. His Sandia project work is developing technology for recycling spent rinse waters from semiconductor wet benches.
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