Saving the most complex for last

by Richard A. Matthews

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This is the last in a series of columns outlining the new ISO Global Cleanroom Standards. This month we cover the newest, probably the most complex and perhaps the most far-reaching subject—molecular contamination.

ISO 14644-8
TITLE: Cleanrooms and associated controlled environments—Part 8: Classification of airborne molecular contamination.

SCOPE: Covers the classification of molecular contamination in terms of airborne concentrations of specific compounds or chemicals and provides a protocol that includes test methods and analysis for concentrations between 10° and 10-12 grams/cubic meter.

STATUS: Currently being developed by ISO/TC209 Working Group 8. It is expected to be published as a Draft International Standard (DIS) by September 2001.

Molecular contamination is the new area and era of concern in the contamination control community. Today we have very good control of particle contamination down to 0.1 µm. This is the result of years of improving design, control and practical experience.

Driven by the demand for ever-smaller microelectronic components and the real-size world of biotechnology forces the cleanroom community to be concerned with contamination caused by non-particles, that is, molecules.

ISO/TC209 decided in 1998 to add this contamination source to its family of contamination control standards. This decision was predicated on the fact that this is a fast-changing and complex contamination source, but it is possible to measure, verify and analyze this contamination on a repetitive basis. ISO-14644-8 is the base document for controlling molecular contamination in cleanrooms and associated controlled environments.

It includes the special requirements of Separative Enclosures (see ISO 14644-7) such as minienvironments, isolators, glove boxes and clean hoods.

Often molecular contamination can only be identified by its after-effect, which is surface molecular contamination; that is residue or resultant chemical or molecular change on a surface.

Airborne molecular contamination (AMC) is the presence in a cleanroom atmosphere of chemicals (non-particle) in the gaseous, vapor or liquid state, which may have a deleterious effect on a product, process or analytical instrument.

Surface molecular contamination (SMC) in a cleanroom is the presence on the surface of a product or analytical instrument of chemicals (non-particle) in the gaseous, vapor or liquid state, which may have a deleterious effect.

Outgassing is gaseous products released from a material under specified conditions of temperature and pressure.

ISO 14644-8 provides a formal classification system of AMC. This system has an ISO descriptor, which is as follows:
AMC.ISO Class N:a:b; (c); (d)
N = the logarithmic index of concentration expressed in grams/cubic meter
a = the type of compound (acid, base, organic, inorganic)
b = the specified measuring method (sampling and analytical)
c = the optical extension for a particular species
d = the optional extension to include elapsed time

Let's review a few examples:

  1. “AMC: ISO Class -6:A:IMP-IC; (HCl); (-)” Translated this is an airborne concentration of HCl (hydrogen chloride) acid of 10-6 grams/cubic meter, sampled with an impinger (IMP) and analyzed with ion chromatography (IC) and no elapsed time period (-).
  2. AMC, ISO Class -5:0:SOR-GC-MS; (DOP); (2016 post operational) Translated this expresses an airborne concentration of DOP which is a species of organic (o) at 10-5 grams/cubic meter sampled with a sorbent tube (SOR), analyzed by gas chromatography mass spectroscopy (GC-MS) at an elapsed time of 12 weeks expressed as 2016 hours post operational, i.e., after 12 weeks of cleanroom operation. There is also a provision for measuring surface molecular contamination and this is expressed in the next example.
  3. SMC: ISO Class -8:0: DIFF-GC-MS; (DOP); (24)

The translation here is surface concentration of DOP (an organic) 10-8 grams/cubic meter after 24 hours exposure as sampled with passive diffusive sampling (DIFF) analyzed using gas chromatography mass spectroscopy (GC-MS).

All this may seem overwhelming, but it is really based upon sound scientific principles and measurement. ISO 14644-8 as currently under development has been well organized to give guidance for developing a sound procedure assessing the parameters affecting airborne and surface molecular contamination in a cleanroom or other controlled environment.

The AMC and SMC classifications for molecular contamination are entirely separate from the classification of air cleanliness found in ISO 14644-1 for particle air cleanliness.

ISO 14644-8 provides a detailed checklist of potential cleanroom-related molecular contamination sources. In addition, it lists typical contaminating chemicals and substances. There is also a listing of typical methods for the measurement and analysis of molecular contamination both passive and dynamic.

Five different sampling instruments and 16 different analysis methods are shown, and these are by no means all the options available. However, the instruments and methods must be measurable, verifiable and repeatable.

The idiosyncrasies of barrier technology, as found in isolators, minienvironments, glove boxes, clean hoods and the impact of molecular contamination therein, is addressed in ISO 14644-8.

The last 18 pages of this document clearly spell out standard evaluation methods for acids, bases, organics (condensables) and inorganics (dopants).

The control of molecular contamination is a field still in its infancy but growing and changing rapidly. It is important to our future. ISO 14644-8 does not recommend any specific control program or device. Rather it provides the means for identifying and assessing the amount of molecular contamination present.

Molecular contamination can also be controlled by chemical filtration systems, dilute chemistry, alternate chemistries, event control (spills or product handling) and common sense when sources are identified and quantified.

Because ISO 14644-8 is still in the development stage, copies for public review and comment are expected to be available from the Institute of Environmental Sciences and Technology, 940 East Northwest Highway, Mt. Prospect, IL 60059 in September 2001 when ISO 14644-8 goes out for DIS vote.

Richard A. Matthews is founder of Filtration Technology Inc. (Greensboro, NC) and president of Micron Video International. He is chairman of the International Organization for Standardization Technical Committee ISO/TC209 “Cleanrooms and associated clean environments.”

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