Importance of controlling airborne molecular contamination impacts filter manufacturers and media suppliers
By Robert McIlvaine and Karen Vacura, The McIlvaine Company
Airborne molecular contamination (AMC) is non-particulate chemical contamination in the form of vapors or aerosols that has a detrimental effect on a product or process. The need for AMC control in cleanrooms continues to grow as technology advances.
A necessity in the lithography process, AMC control is becoming more important in other cleanroom processes. As processing moves to faster speeds, manufacturers are seeing increasing sensitivity to contaminants, affecting yields.
Some common reasons for instituting AMC control are to prevent corrosion during processing and to protect reticles during transport or storage, according to Christopher Muller, technical director for Purafil, Inc., a leader in the engineering and manufacture of gas-phase air filtration media, systems and monitors. In leading-edge fabs, a large amount of outside air is pulled in for ventilation and pressurization purposes, and an even greater amount of air is recirculated, all of which needs to be monitored for molecular contamination. Additionally, reticles are at risk of damage due to haze formation from molecular contamination.
The International Organization for Standardization (ISO) has recently published ISO 14644-8, Cleanrooms and associated controlled environments–Part 8: Classification of airborne molecular contamination. This part of ISO 14644 covers the classification of airborne molecular contamination in terms of airborne concentrations of specific chemical substances (individual, group or category) and provides a protocol for test methods, analysis and time-weighted factors within the specification for classification. The document is designed for use in a wide range of industries, including microelectronics, pharmaceuticals and medical devices.
Applications for AMC control
AMC sources fall into two categories: internal and external. Some internal sources could be chemicals used in the manufacturing process, accidental spills, and off-gassing of cleanroom components. External sources vary by plant location and include factory emissions and auto exhaust. For example, in Taiwan, one fab’s exhaust can be the next fab’s intake. Proximity to farm fields can also raise the need for AMC control. In general, outside pollutants are removed from the make-up air, and indoor-generated contaminants are removed from the recirculating air.
AMC control is needed at different points in a facility, including: make-up air; re-circulated air in the ballroom, minienvironments and enclosed areas; fan filter units for localized control; and exhaust air. With the decrease in wafer carrier sizes, certain processes may only require AMC control in a smaller enclosed area, rather than for the whole ballroom. In a 200,000-square-foot semiconductor cleanroom, about 30 to 40 percent of the area is devoted to lithography processes. Only this area, roughly 60,000 to 80,000 square feet, needs AMC control.
According to Michael O’Halloran, director of technology at CH2MHill, engineering firms look at specifics for each application to find the most efficient and economical solution, balancing near-term and future costs. For example, engineers try to determine whether a contamination event is temporary, perhaps due to outgassing of new construction, or whether it’s a potential long-term problem.
CH2MHill uses a virtual airflow-modeling program, Computational Fluid Dynamics (CFD), to monitor contamination during process operations. This tool allows for the characterization of overall cleanroom airflow patterns, pressurization and temperature effects during the design phase of the cleanroom. The general approach is to improve the overall airflow patterns by minimizing recirculation zones that collect and transport contaminants, and to select areas for AMC control. CFD can also be used to track down the source of contamination in an existing cleanroom.
O’Halloran explains that the incorporation of a minienvironment into the lithography tool is part of a standard setup. Some tools require the background air to be controlled to a certain level in the cleanroom, and others rely on the minienvironment of the tool to do it. The life of the tool’s AMC filter is determined by the extent of challenges it receives: If the cleanroom AMC is controlled, the minienvironment filter will have a longer life.
Solutions and products
When choosing a system for AMC control, the most important consideration is what types of gases affect the process and personnel, and at what level. In general, AMC is specified to be less than 1 part per billion (ppb) over the service life of the system.
AMC control requires a combination of media and filter types, including prefilter usage, according to Purafil’s Muller. Types of common AMC filtration systems are:
- Adsorption/Chemisorption: Adsorption uses granular activated carbon and/or activated aluminas. The removal capacity is directly related to total surface area. Chemisorbent systems use adsorption and specific chemicals added to activated carbons and/or aluminas.
A reaction occurs with contaminants to form stable chemical compounds that either bind to the media or are harmlessly released into air. - Bonded media panels use granular adsorbents, such as activated carbon, bonded and formed into monolithic (single-piece) panels.
- Ion exchange systems use synthetic polymers with positive or negative charged sites on pleated membrane or spongelike, flat sheets. This type of contamination control is mainly used in liquid applications but is finding specific niches as AMC control, such as for ammonia.
Practically all chemical filtration media today are manufactured from activated carbons and/or alumina. Specific chemical additives are utilized to impart special characteristics to the media. The target contaminant determines what chemicals are impregnated on the carbon or alumina. Potassium permanganate is a common, broad-spectrum chemical used almost exclusively on activated alumina since it cannot be effectively used with activated carbon. Pore size and structure of activated carbon can also affect how impurities are adsorbed.
Blended media for multiple contaminants are also used. For instance, alumina impregnated with potassium permanganate used in conjunction with plain or impregnated granular activated carbon provides a very broad-spectrum, gas-phase air filtration system. However, many manufacturers prefer to focus their AMC control on the specific contaminant type. Also, blended media have a shorter service life due to the reduction in the quantity of each media as compared to a system employing both media in individual stages. Filter life is based on capacity, with variables of temperature, airflow velocity, and contaminant.
Chris Hicks, a sales representative for Calgon Carbon Corp., explains that activated carbon works for most applications. Calgon manufactures several hundred carbon products to address almost any contaminant, and also offers activated alumina products. Hicks points out that AMC control needs to change as new exhaust regulations are passed. There is currently a shift toward adsorption from combustion. Calgon Carbon is starting to look at workstation filtration, targeting exhaust at site generation of contaminants. In this application, a separate module not integral to lab hoods would be employed.
Major suppliers of AMC filters are M&W Zander, Camfil Farr, and Purafil. American Air Filter (AAF) has developed a line of chemical filters as well. Asian companies Takuma, Takasago Singapore, and Taiwan Nitta also supply chemical filters, primarily aimed at minienvironments.
Cost considerations
Purafil’s Muller explains that costs can differ widely with application needs and whether the system is customized or integrated into an existing system. Chemical filter costs can range from $40 to $50 for a 24-inch x 24-inch x 2-inch outside
filter to $3,000 for a 24-inch x 24-inch x 12-inch tool filter. Chemical filters can have a higher pressure drop than HEPA filters, so increased energy costs should also be considered.
Monitoring filter life is another cost issue. According to Lighthouse Worldwide Solutions, depending on what chemicals are being monitored and how many locations are sampled, the cost can range from $3,000 for a single sensor to $400,000 for an entire system that samples multiple locations.
Performance and service life of filter systems differ with contaminant gas. CH2MHill’s O’Halloran says that monitoring must be a continuous process since failure load of chemical filters can be very sudden, over a span of just days or weeks. One efficient monitoring technique is to take a sample of the filter and test it; another method, although with less ability to predict future failure, is to monitor filter discharge.
AMC market forecast
Revenues for AMC removal have been projected based on the expected penetration in the microelectronics industry. A high penetration of the semiconductor industry and lower penetration of other microelectronics applications has been assumed. Other factors include filter life and cost.
Definition of the product is difficult. The filter element itself could be a stand-alone product or part of the total filter system. In either case, there is some additional ductwork, fan and housing cost for the extra treatment step. Forecasts for both filters and systems have been calculated.
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AMC filter sales are projected to rise from $50 million in 2006 to $67 million in 2010, with the bulk of the sales to the semiconductor industry (see Fig. 1).
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AMC system sales are projected to rise from $100 million to $136 million during the same period (see Fig. 2).
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By 2010, China will account for 7 percent of the sales but will also be increasing purchases at double-digit annual rates. Asian countries will account for the majority of AMC filter purchases (see Fig. 3).
Robert McIlvaine is president and founder of The McIlvaine Company in Northfield, IL. The company first published Cleanrooms: World Markets in 1984 and has since continued to publish market and technical information for the cleanroom industry. He can be reached at [email protected].
Karen Vacura is the air filtration market editor for The McIlvaine Company. She can be reached at [email protected].
References:
- Polen, Morgan, Peter Maquire. “AMC Measurement and Control,” Lighthouse Worldwide Solutions, application note, http://www.golighthouse.com.