Contamination Control In And Out of the Cleanroom: Trapping Airborne Molecules: Molecular Filters, Gas Purifiers - Part 1

Contamination Control In And Out of the Cleanroom: Trapping Airborne Molecules: Molecular Filters, Gas Purifiers - Part 1
By: Barbara Kanegsberg and Ed Kanegsberg
October 2009

“AMC filters don’t work.” “We don’t need to trap AMC’s.” These facile sentiments impede progress in critical applications.

Certainly, being proactive with Airborne Molecular Contamination (AMC) is essential. Understanding, detecting, and eliminating or reducing AMC sources should be the first line of defense.¹ Sometimes, however, AMC must be trapped; you have to “head it off at the pass.” Successful removal of molecular contamination from air and gas streams requires knowledge of the chemical nature of the contaminating molecules.

Why are AMC trapping devices not ubiquitous? The field of molecular filtration and purification is still emerging from the shadow of emphasis on particle control. Further, many industry sectors still do not fully appreciate the importance of molecular level process control and the distinctions between particle removal and molecule removal.

While specific applications are distinct, AMC removal and gas purification, including the preparation of Clean Dry Air (CDA), have many common processes and suppliers. Both involve removal of unwanted molecular species before they reach the surface of the product.

MOLECULAR REMOVAL IS MORE COMPLEX THAN PARTICLE FILTRATION
For particles, the primary discriminator is particle size. A particle filter acts somewhat like a window screen to let small particles and gases through and block larger particles. Granted, High Efficiency Particle Air (HEPA) filters are more complex than a window screen, and they can trap or impede particles both larger and smaller than the pore openings between filter fibers. However, standard HEPA filters do not remove molecular contaminants that are several orders of magnitude smaller than the size rating for a HEPA filter. For AMC filtration, rather than removing contaminants on the basis of size alone, the chemical nature of the contaminant becomes very important.

TERMINOLOGY
The fact that molecular filtration is much more complex than particle filtration is reflected in the terminology. The terms filtration and purification are both widely used; some experts are adamant about the appropriateness of one term over another. Some use filters for devices that physically separate gas components but do not change their chemical nature as distinguished from purifiers for devices that alter the chemical being removed. We tend toward the view² that a filter is a device to perform purification. Other terms, such as trap, sieve, desiccant, or getter, may be used to describe certain filtering or purification functions.

STANDARDS
ISO 14644-8, a more encompassing descendent of the SEMI F21-1102 standard, creates eight categories for molecular contaminants (Table 1). The categories reflect different mechanisms by which contaminants can be damaging to a product.

It is noteworthy that the Institute for Environmental Sciences and Technology (IEST) has recently released a first-edition document³ that provides guidelines on the design of filter systems to eliminate trace amounts of AMC.

FILTER LIFETIMES
Those who have worked with HEPA filters exclusively require a paradigm shift in managing AMC filters or purifiers. A properly installed HEPA filter, with appropriate pre-filters, has a very long lifetime. The energy needed to force air through a HEPA filter increases with use. This can be monitored by tracking the pressure drop through the filter and provides a metric to determine when energy costs make replacement economical. In addition, if the pressure drop gets too large, seals can fail and unfiltered air bypasses the filter.

In contrast, most molecular filters or purifiers are consumables with finite lifetimes. With many, there is little or no indication of loss of efficiency. An AMC filter or gas purifier may maintain the design level of efficiency until essentially all active sites have been exhausted, at which point the device rapidly stops working. Therefore, most users employ a periodic replacement schedule.

APPLICATIONS
Applications for molecular filters or purifiers are quite varied. Many of the most stringent requirements still come from the semiconductor and related industry areas such as photovoltaic devices. However, there is increasing interest in other critical applications such as aerospace and precision optics. Examples include prevention of degradation of surfaces that are to be joined or coated and control of outgassing into confined spaces. Additional perhaps non-traditional applications for AMC filtration/purification include conservation of art and documents, and achieving good indoor air quality and odor control in schools, commercial buildings, and airports.

Some purification systems not only provide high degree of purification but also include a method for certification of purity of either CDA or of an individual gas. One system⁴ contains four sampling traps. The gas to be certified is drawn through a set of three traps/purifiers/filters to respectively trap acids, bases, and hydrocarbons/refractory gases. The traps are subsequently submitted for analysis, the acid and base traps by ion chromatography and the hydrocarbons/ refractory gases by thermal desorption gas chromatography. Analysis of the control filter/trap, installed downstream of the first set of traps, allows the determination of the percent of capture. For example, if the first set of filters trap all impurities, the control would analyze as “empty.”

MOLECULAR TRAPPING MECHANISMS
Molecular trapping devices do work. However, it is essential to select the appropriate trapping device and to establish and conduct a maintenance/replacement schedule.

Given the diverse nature of molecular contaminants, there is no universal AMC filter or purifier.

Next month: We will look at several mechanisms for filtration or purification.

Acknowledgements
The authors acknowledge the helpful comments of Mike Hoke, Matheson Tri-Gas, Inc.; Cristian Landoni, SAES Pure Gas, Inc.; Mark Stutman, Camfil Farr; and Gerald Weineck, Donaldson Company, Inc.

References

1. B. Kanegsberg and E. Kanegsberg, “Contamination Control In and Out of the Cleanroom.” Controlled Environments Magazine, June, Jul/Aug., and Sept. (2009).
2. G. Weineck, Donaldson Company, Inc, personal communication.
3. “Design Considerations for Airborne Molecular Contamination Filtration Systems in Cleanrooms and Other Controlled Environments”, IEST-G-CC035.1, available through www.iest.org.
4. C. Landoni, SAES Pure Gas Inc., personal communication.