BY Chris Muller, Purafil, Inc.
Microelectronics manufacturers have become much more sophisticated in their knowledge and understanding of airborne molecular contamination (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 has grown, so too have their requirements for an AMC control system.
Some manufacturers’ concerns about the proper selection of an AMC filtration system have become so acute that they are being reflected in their control specifications. One manufacturer may call for a minimum 90-percent removal of target contaminants, while another will set absolute AMC control limits of one part per billion (ppb) or less. Still another may require a minimum of one year between filter changeouts. As tight as these requirements may seem individually, there are current specifications that call for all three of the above criteria to be met.
Compounding this situation, some manufacturers are trying to find a “one filter fits all” solution for AMC control, demanding that a single filter should meet all of the control criteria for all contaminants of concern. This is somewhat quixotic thinking considering that, comparatively, AMC filtration systems are expected to accomplish in one or two stages what may require as many as six stages of particulate filtration to achieve relative to specified cleanliness levels. A lack of physical space in the air handler in which to add additional filtration, insufficient pressure drop available to handle multiple stages of filters, increased energy costs, and cost constraints in general have all been cited as reasons for this quest for a single-filter solution.
AMC concerns
In much of the world today, ambient (outdoor) air contains levels of molecular contaminants high enough to cause problems in the cleanroom. The form AMC control takes will therefore differ depending on where it is being applied in a facility.
1. Make-up air systems will see primarily atmospheric contaminants if the outside-air intake locations are chosen carefully. Zero-downtime systems should be considered for make-up air handlers. A balance should be struck between the desired level of AMC control versus pressure drop versus service life.
2. Recirculation air systems require that AMC control be chosen based on functional area requirements.
3. Minienvironments with self-contained chemical contaminant-control systems can significantly reduce the occurrence of odors related to production processes. Added benefits include increased protection for the product and the reduction of the total exhaust requirements.
4. AMC filtration added to fan filter units (FFUs) can provide localized control of specific contaminants.
5. Exhaust air systems generate a significant number of complaints from neighboring facilities due to nuisance odors from exhaust abatement equipment. Continuous monitoring and regular maintenance is warranted. Careful location of exhaust stacks, compliance with environmental regulations, and dispersion modeling is required for all production facilities.
Air cleaning
Along with removal of contaminant sources (emissions control) and ventilation with clean dilution air, air cleaning is one of three AMC control techniques most commonly applied in the cleanroom. Source control for outdoor air contaminants is often not feasible, practical, or even possible, therefore, ventilation control would be the next option. However, this is not practical in many cases because the use of large amounts of outdoor dilution air is neither cost-effective nor energy-efficient. Further, bringing in additional quantities of outside air could result in substituting internally-generated contaminants for those with sources outside the cleanroom. In areas with unacceptable outdoor air quality, neither source nor ventilation control can prevent the introduction of contaminants into a facility, and therefore, air cleaning must be employed.
Make-up (outdoor) air systems must typically be designed to control SOx, NOx, ozone, VOCs, and some site-specific contaminants such as chlorine, organophosphates, and ammonia. AMC filtration equipment in recirculation systems must be designed to remove a wide array of organic and inorganic sulfur and nitrogen(-containing) compounds, hydrocarbons and other VOCs. As a rule, organic compounds are the most abundant types of AMC found in these facilities.
 Figure 1. AMC filtration system with prefilter (left), chemical filter (center) and final filter sections (right). |
An idealized AMC filtration system for use in a make-up air handler is shown in Figure 1. The prefilter section removes unwanted particulate matter and serves to protect the chemical filtration media. The chemical filter section removes unwanted gases and odors and protects production processes, materials, equipment, and personnel. The final filter section removes any remaining particulate matter and serves as a “polishing” filter upstream of the HEPA and ULPA filters.
The optimum system will contain multiple stages of AMC 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.
Chemical filtration application areas would include air handlers introducing outdoor air into the facility as well as those being used in air handlers serving production areas. Fugitive emissions from processes, and chemical leaks and spills, can be addressed using one or more gas-phase air filtration media. A schematic of a cleanroom showing possible locations for AMC filtration systems is shown in Figure 2.
Given all of the above, how is the contamination-control professional expected to choose the appropriate AMC control options for a particular application? To address this situation is exactly the reason one of the IEST’s contamination-control working groups was formed.
IEST Working Group WG-CC035: “Design Considerations for Airborne Molecular Contamination Filtration Systems in Cleanrooms”
The purpose of this Working Group is to develop a Recommended Practice (RP) that will provide a framework for discussion between users, contractors, and manufacturers concerning the specification, selection, and use of air filtration systems to remove AMC from the air supplied to cleanrooms, clean zones, clean-air devices, and other controlled environments.
This RP will describe important areas of concern when considering AMC filtration systems. It covers various applications, potential uses within the cleanroom, filtration methods, materials of construction, performance comparison and follow-up assessment, secondary impacts of the filters on the rest of the HVAC system, and exterior impacts on the filtration system (including environmental conditions, upsets, spills, etc.). It will establish the types of information required to design and implement an effective AMC filtration system. While this document will be targeted at cleanroom facilities, it could also form a framework for other applications as well.
Because of the large number of variables, this document will not attempt to recommend specific AMC filtration systems or filtration media for each application. This RP will not establish a test standard, though several applicable standards will be referenced. It will not address microbiological or radioactive issues/applications or emergency response systems. This RP will be concerned mainly with molecular contaminants that are present in trace amounts (typically << 1 ppmv) as opposed to pollution control, for which different types of intervention are necessary. This RP will not deal with respirators as there are already accepted test methods and standards for these devices.
IEST-RP-CC035.1 will provide the basic information necessary to develop and implement an effective AMC control program, and will be applicable to a large number of end users (those with cleanroom facilities) as well as architectural and engineering design firms, consultants, and suppliers.
Conclusions
The need for AMC control can range from odor control to safety requirements to process yield improvements. The level of required performance will determine the type of filter system acceptable for the application.
Air cleaning is often used as an adjunct to source control and ventilation. An AMC filtration system as an integral part of the cleanroom’s HVAC system can effectively reduce AMC to levels that are at or below the level of detection for the monitoring techniques employed. Properly applied, AMC filtration also has the potential for energy savings.
IEST-RP-CC035.1 will provide the information necessary to properly design, install, and maintain AMC filtration systems that will be able to effectively and economically remove essentially all chemical contaminants of concern from the cleanroom environment. When AMC control is employed in a cleanroom manufacturing environment, one gains the dual benefits of process/product protection as well as overall improved air quality. Although both can benefit the bottom-line, the latter can have even more far-reaching implications when worker health and productivity are considered. III
Chris Muller is Technical Services Manager for Purafil, Inc., a manufacturer of AMC filtration media, equipment, and monitoring instrumentation. He is a senior member of the IEST and is a member of several Working Groups. He can be reached at [email protected].