Manufacturers have become much more sophisticated in their knowledge and understanding of AMC and its effects in the cleanroom. They have a better general understanding of where AMC control should be applied and why, and as their knowledge of AMC-related problems have increased, so too have their expectations for an AMC control system.
We’ve previously discussed airborne molecular contamination (AMC) in terms of its classifications, monitoring, and control considerations and technologies, as well as AMC filter types and applications. This final installment will focus on specifying an AMC control system and final considerations for AMC control.
SPECIFYING AN AMC CONTROL SYSTEM
Manufacturers have become much more sophisticated in their knowledge and understanding of AMC and its effects in the cleanroom. They have a better general understanding of where AMC control should be applied and why, and as their knowledge of AMC-related problems have increased, so too have their expectations for anAMC control system.
In fact, some manufacturers’ concerns about the proper selection of a control system have become so acute that it is reflected in their control specifications. One manufacturer may call for a minimum of a 90% removal of target contaminants, while another will set AMC control limits at 1 ppb or less. Still another may require that the system must last a minimum of one year between filter changeouts. As strict as they may seem individually, thereare manufacturers that require all three of the above criteria to be met.
Compounding the situation, some manufacturers insist on trying to find a “one filter fits all” solution for AMC control, demanding that a single filter should meet all of their control criteria for all contaminants of concern. However, chlorine requires one type of filter, ammonia another, and organic compounds still another. The type of filters/systems used for toxic gases wouldnot be the same as odor control.
“Are you looking for high efficiency or long service life?” “Do you require absolute control of one contaminant or relative control of a group of contaminants?” These are only some of the questions that an AMC control system designer must consider, even though his customer may not understand the implications of not takingall of this into consideration.
Specification by Removal Efficiency
The removal efficiency of an AMC control system can be considered the fraction of a single contaminant or group of contaminants that are removed eitherby physical or chemical means.
Many manufacturers are able to provide test data for their systems that show removal efficiency over time. However, this testing has been performed almost exclusively under accelerated conditions using highcontaminant challenge concentrations that can be up to three to four orders of magnitude higher than what would be expected under actual use conditions.
Although realistic extrapolations of filter efficiency can be provided, many manufacturers are now requiring low-level gas challenge testing for efficiency ratings of the filters (see Figure 1).1 This is the best way to gauge filter performance; however, testing of this sort is more complicated, takes a long time to complete, and is much more expensive to perform.
One of the main problems in trying to provide an efficiency rating for a particular contaminant/filter combination is that any rating has to have a time component specified along with it. If a specification calls for a 90%-minimum removal efficiency, the end user is expecting that to hold true over the life of the filter.
Most tests, however, are only run long enough to provide an “initial” efficiency, and the test is stopped after 1 hour, 8 hours, 24 hours, and so on. All this does is provide information on how well the filter may work under a specific contaminant load, but does not provide or predict an indication of performance beyond the stated test period.
An examination of the efficiency curves can provide some additional indication of future performance; however, it still cannot guarantee performance under actual-use conditions. Further, a filter that performs well against a single contaminant, such as ammonia, can perform poorly when gas mixtures are considered.
Specification by Contaminant Limits
Some manufacturers have investigated specific AMC-related process problems well enough that they have been able to set specific control levels for one or more contaminants. Common control specifications call for less than 1 ppb ammonia in lithography bays or less than 1 ppb chlorine or hydrogen fluoride (HF) in met-allization processes. Regardless of the ambient levels of ammonia,chlorine, or HF, the AMC control system is expected to keep the controlled environment at or less than 1 ppb over the service life of the system. Depending on ambient levels, this could require a minimum working filter efficiency of 50%, 80%, 95%, and so on. If transient, high-level episodes are probable, filter efficiencies greater than 99% may be required to maintain a 1-ppb level coming out ofthe AMC control system.
Specification of Service Life
Cost-of-ownership is always a major consideration when applying AMC control and can unfortunately be the primary factor when the final purchasing decision is made. Budget cycles, production schedules, capital expenditures, or simply a customer’s preconceived notion of how long a filter should last canbe the basis for specifying filter life.
Testing to determine working removal capacities of a certain AMC control product for a specific contaminant follows the same basic protocol as for efficiency testing. Using a known contaminant concentration at a specified airflow (typically the filter’s maximum rated airflow), and running to a specific efficiencyendpoint, one can calculate the amount of contaminant that has been removed.
The media’s removal capacity is reported as a volumetric capacity (g/cc) or as a weight percent. This can then be used to estimate media consumption rates under a given set of conditions and provide an estimate of service lifefor a specific filter type.
Capacities have to be determined for each contaminant of concern in order to provide a total consumption rate for a particular application. This is not practical, however, due to the uncertainty of the total contaminant load, the nature of the contaminants in question and any associated safety concerns, as well as the time and costs involved. Further, the effects other contaminants in the cleanroom may have on the AMC control system have to be accounted for.
Using observed removal capacities are useful in providing estimates of service life, and if a conservative approach is taken, these estimates can be valid tools for the determination of filter service life (see Table 1).2
Ultimately, filter changeout schedules will come from experience. Actual-use conditions often dictate that different media types in individual AMC control systems will need to be replaced at different intervals to maintain optimum performance.
This is not the answer that manufacturers want to hear because they want to be able to set maintenance schedules and budgets; however, the reality is that one cannot predict, much less guarantee, filter service life from testing on one or two target contaminants while not knowing the total contaminant loadthe system will actually see.
Although the specification of an AMC control system is a difficult undertaking, given the proper considerations, one can be successful in specifying an effective and economical solution for most applications. One cannot go in with preconceived notions about how a particular system will perform under a given set of conditions, rather one should determine what they ultimately want to achieve by the installationof such a system.
These five points should always be considered in the design and specification of an AMC control system:
1. Does AMC pose an immediate health threat to cleanroom personnel or is it primarily an odor control issue?
Personnel protection requires 100% control of the offending contaminants whereas odor control could be achieved with a system operating at an overall efficiency of 50% or less. Process protection requires very high performance, but that does not mean absolute control. Many specifications today call for removalefficiencies of at least 90% and this is realistic to expect.
2. Accept filter performance testing for what it truly is. That is, a comparison of different systems being offered for a particular application. Efficiency or capacity test data for a single contaminant cannot be used as an absolute predictor of system performance. It can provide an indication of the relative performance differences between competitive systems.
3. Request performance data for contaminant concentrations at or near expected use conditions if at all possible. If system performance and/or filter life estimates are based on accelerated test results, consider limiting gas challenge concentrations to no more than two orders of magnitude above what would be expected under actual use conditions.
4. Filter service life should not be a part of a performance specification. Filter life estimates should be required for all systems being considered, but again, these should be used as relative comparisons and not absolute values. There is no practical way to provide service life estimates without having performance data for the total contaminant load and not just a handful of target contaminants. Monitoring of filter performance using direct gas monitoring, reactivity monitoring, or by using grab samplers (impingers, adsorbent tubes, witness wafers, coupons, etc.) should be started along with system start-up and should continue as long as the AMC control system is in place.
5. Use a multi-stage AMC control system. Just as particulate control in cleanrooms requires several stages of filtration to achieve the desired cleanliness level in the protected space, the same should be considered for an AMC control system. A “prefilter” stage should be used to remove as much of the “junk” AMC as possible. This would help protect and preserve the “final filter’s” ability to remove those contaminants of concern with good efficiency and capacity. If the types of contaminants targeted for control require different filters, serious consideration should be given to individual stages for each filter type. Blended media may be used, but at a trade-off in service life.
A properly designed, installed, and maintained AMC control system can fairly easily achieve the removal efficiencies required for specific target contaminants. How long a system will meet specific performance criteria depends on the average and peak values of ALL contaminants present, which must be considered in thefinal design of the AMC control system.
The optimum system will contain multiple stages of filtration to provide high initial and average removal efficiencies as well as an acceptable service life. It will involve higher front-end costs, but will ultimately lead to lower operating costs and a lower likelihood of AMC making its way into the cleanroom and into critical process areas.
The authors wish to thank BBA Fiberweb – AQF of Charlotte, North Carolina and Miller Nelson Research, Inc. of Monterey, California for supplying some ofthe test data used in this article.
- Miller Nelson Research, Monterey, California, “Purafilter Gas Challenge Test Results,” proprietary data, Purafil, Inc., Do-raville, Georgia, 2001-2003.
- Chris Muller, “Specifically AMC: Guidelines for specification of AMC control,” Cleanroom Technology Magazine, April, 2004.
Christopher Muller received his BS in Applied Biology from the Georgia Institute of Technology, with post graduate education focusing on industrial engineering.He is currently the Technical Services Manager for Purafil.He has written and spoken extensively on the subject of airborne molecular contamination (AMC) control and reactivity monitoring in cleanroom applications and has published more than 50 papers and articles as well as four handbooks.He is a senior member of the IEST, and he chairs ASHRAE’s Standard Project Committee 145P.Muller is also a member of ISA’s SP71 committee on Environmental Conditions for Process Measurement and Control Systems.He is involved in updating SEMATECH’s International Technology Roadmap for Semiconductors and serves on the YieldEnhancement Technical Working Group.
Brad Stanley is a graduate of the Georgia Institute of Technology with a BS in Chemical Engineering. Brad currently works as a Senior Application Engineer with Purafil assisting in research and development pertaining to the manufacture, testing methods, and application of Purafil’s gas-phase filtration media and atmospheric monitors. Brad has worked in this capacity for five years at Purafil and serves as a technical contact for applications pertaining to the use of Pu-rafil’s chemical media and systems.He has authored numerous papers and presentations on subjects including indoor air quality, corrosion of microelectronics, and atmospheric monitoring methods for controlled environments.Brad can be reached at 800-222-6367 or email@example.com
Purafil, Inc.provides quality clean air solutions.