by Sheila Galatowitsch
Concern over molecular contamination is the driving force behind key innovations in air-filtration media
It's the heart and soul of all air-filtration systems: the material of the filter itself. And like the rest of the cleanroom industry, it's changing.
Innovations in filter media spring directly from grass- root users who request certain specifications from filter manufacturers. The filter makers then turn to the media suppliers for solutions. In recent years, this triad has crafted improvements to conventional microfiberglass media, launched an alternative membrane composite media and made significant progress toward understanding and controlling airborne molecular contamination (AMC).
“The past two to three years have been a wild ride,” says Pat Paquet, cleanroom market manager for filter manufacturer American Air Filter (AAF; Louisville, KY). Processes in both microelectronics and non-electronics cleanroom applications have made quantum leaps, pushing filter media advances and overall market growth. The end result is that users have more choice, not only in the types of contamination to be removed, but in the performance levels and cost of various media.
While concern over molecular contamination has motivated most innovations, media for particle filtration has not been neglected. Here's a look at what's new in microfiberglass, membrane and synthetic media for particle and molecular contamination control.
Microfiberglass media: Improving alpha
“It's been said more than once that the war on particulate filtration has been won,” says Joe Doherty, marketing manager for air filtration at Lydall Filtration/Separation Group (Rochester, NH). Yet as microfiberglass-based HEPA and ULPA filtration efficiencies continue to rise (some ULPA filters offer as much as 9-nines efficiencies), Doherty has seen a decrease in the levels of filtration efficiencies that users are requesting.
High-efficiency microfiberglass media, like this rolled material from Lydall, helped cleanroom users win the war on particle contamination.
“Four or five years ago, users were interested in filters that offered the highest possible levels of particulate filtration99.999999 percent or higher. Today there is a much greater focus on understanding the levels of filtration that are actually required in different areas within a cleanroom,” says Doherty. “Companies are analyzing the levels of filtration that are necessary to improve product quality, yield and product costswithout overspecifying efficiency. Higher levels of filtration typically mean higher operating costs in the form of more costly air-handling systems and higher levels of energy consumption.”
This emphasis on controlling operating costs has led Lydall and other media suppliers to continuously improve the so-called “alpha” of microfiberglass media. “Alpha” is an industry term that expresses the relationship between filter pressure drops and particle removal efficiencies. The idea is to maintain or boost efficiency levels of the filter material while lowering its resistance to airflow and lowering pressure drops to the filter.
The actual techniques that contribute to alpha enhancements are proprietary secrets, but all are designed to garner more airflow and more clean air with less electricity and smaller blowers. Lydall offers high alpha versions of its HEPA and ULPA media, which deliver up to 20 percent lower pressure drops than standard media.
Reducing the cost of air filtration is especially attractive to the newest users of traditional microfiberglass mediathe food processing industry. “Food processors don't have set air cleanliness level guidelines and policing as does the pharmaceutical and electronics industries, but that will change,” says Paquet. “This market is volatile and growing, particularly in the face of concern over potential terrorist-inspired biological threats.”
Media makers are also exploring new fibers, binder chemicals and processing methods for microfiberglass-based media. But, increasingly, they are turning their attention to a potential replacement for microfiberglass: membrane composites.
The membrane alternative
Media structured from expanded polytetrafluoroethylene (ePTFE) emerged several years ago when semiconductor manufacturers became concerned about boron leaching out of microfiberglass-based filters. Boron in borosilicatea silicate glass used in microfiberglass mediais part of the glass-fiber composition. But as boron interacts with the chemicals present in wafer processing, it begins to off-gas and deposit on the wafer itself as a foreign substance. Boron has been identified as a dopant and thus is destructive to wafers processed in minienvironments, wet benches and tooling applications in semiconductor fabs.
ePTFE is a stretched thin-film membrane with a fine pore structure that remains chemically inert in the presence of chemicals and offers high particle filtration efficiencies with relatively low pressure drops. It's generally a multi-layered membrane with scrims protecting the ePTFE inner core. Because it is up to 10 times more expensive per square foot than microfiberglass media, it is presently produced and sold to a “need-only” audience. “But in wafer processing, it's worth the investment,” says Paquet. “And recent tendencies to more resilient media needs from other markets seem to be emerging.”
For now, ePTFE is the only membrane composite material available for cleanroom air filtration. Daikin Corp. and Nitto Denko Corp. in Japan and W.L. Gore & Associates in Elkton, MD, are among the vendors supplying ePTFE media to HEPA and ULPA filter manufacturers. Lydall has recently joined that group with the introduction last year of its membrane composite material.
The Lydall LydAir MC-branded media sandwiches ePTFE between two layers of a synthetic non-woven support structure, which makes the fragile membrane more durable and gives it the physical properties for pleating. “After it is pleated, it is actually more durable than microfiberglass,” says Doherty. “ePTFE also has a higher alpha than microfiberglass media.”
Also last year, Gore introduced the next-generation version of its ePTFE filter media. The new product, called Pristyne UX, has a 99.99995 efficiency rating and a 10 percent lower pressure drop than the previous generation. It was developed in response to requests from large semiconductor users who demanded 6-nines efficiency, says Bill Hanna, Gore's semiconductor market specialist. Most of the time Gore's ePTFE media ends up in filters for semiconductor tool enclosures and cleanroom ceilings.
Another media innovation, low-boron microfiberglass media, also emerged as a solution to the boron offgassing problem. Low-boron microfiberglass has all the physical attributes of borosilicate media that are needed for manufacturing cleanroom filters with current manufacturing processes without the high boron content, says Donna Kasper, division director of IEST's filtration working groups and senior account manager at Hollingsworth & Vose's High-Efficiency and Specialty Filtration Business Unit (West Groton, MA). Hollingsworth & Vose's low-boron media is currently used in microelectronics applications in the Far East and Europe, says Kasper.
Low-boron media is cheaper than ePTFE, but Paquet predicts membrane media will gain greater market acceptance. In three years, AAF's membrane filter business has gone from zero to 20 percent. “We will see a continued decrease in microfiberglass demand versus an equivalent increase in ePTFE demand,” says Paquet. “It may take a decade or more, but ePTFE will ultimately replace microfiberglass media altogether in even the most general of cleanroom applications. It is significantly more resilient than glass during processing and handling, and offers equivalent high efficiencies; plus, it doesn't have the brittleness and dopant offgassing characteristics associated with low-boron media.”
Not everyone agrees that membrane media is poised to entirely replace microfiberglass, however. “There will always be a significant price disparity between fiberglass and ePTFE because the final ePTFE media product requires multiple structural layers, which will always make it more costly,” says Hanna. “The choice of media will always be driven by cleanroom design, and many general applications will seek low-cost alternatives.”
Widespread acceptance of membrane filters hinges on the willingness of filter manufacturers to make capital investments in new equipment that will convert ePTFE media into high-efficiency filters. Current microfiberglass pleating equipment can't handle membrane media. To assist in the adoption of its ePTFE media, Lydall is offering both rolled goods and pleated filter packs that work with installed equipment.
Carbon-loaded AMC media
Media makers focused specifically on AMC control are working on a different contaminant removal model than what is called for in particle filtration. Particles either adhere to or are effectively strained by microfiberglass and ePTFE media, but those mechanisms are ineffective in controlling atomic-sized AMCs, which call instead for a large surface area on which to attract and absorb molecules.
AMC media makers impregnate synthetic non-woven fibers with various functional particlesmostly activated carbon or other extended surface-area materialsthat attract molecular contaminants in the air. These particles can have surface areas as high as 1,500 sq. meters per gram of material. The AQF Technologies division of BBA Filtration (Charlotte, NC), for example, has a proprietary process that uses heat to create a matrix of individual polyester fibers and activated carbon particles. The process generates a three-dimensional network that leaves 99 percent of the surface area available for adsorption.
AQF's current product line of pleatable material includes a general-purpose carbon-loaded media to absorb condensables, such as hydrocarbons, and two other media where the carbons are impregnated with either a base or acid material to remove base and acid compounds from the air. A fourth product targets hard-to-remove hydrocarbons like formaldehyde.
Particles attaching to microfiberglass or membrane filters restrict the area and increase a particle filter's efficiency over time, but that's not the case with AMC filters. As molecular contamination builds up on carbon surfaces, available filtration areas get used up so filters perform worse over time. AMC media efficiencies are highly dependent on the application, but they can reach as high as 99 percent for a period of time, says Matt Middlebrooks, senior research engineer at AQF.
“The end user has to decide what level of contamination is critical to the application. If the air stream has 30 parts per billion (ppb) of ammonia, and the process or product can tolerate a level of 5 ppb before yield is affected, then the filter needs to achieve 83-84 percent efficiencies. If the process can only tolerate 1 ppb, then the filter must have a much higher efficiency, which can be achieved with a higher performance filter system or by changing out a filter more often.”
Efficiencies of fibrous carbon
Most of the carbon in use today for AMC filter media is granular in nature. However, fibrous carbonfiber filaments that have been individually carbonizedis also available and shows promise of providing higher initial efficiencies.
“Because each filament is much smaller than a typical granule, fibrous carbon absorbs materials more quickly,” says Middlebrooks. “But fibrous carbon is very expensive to produce, and it can be brittle, leading to some migration of the particles downstream as you handle the filters.” The use of fibrous carbon also results in a thicker media pad, which creates problems in the construction of the filter.
Still, fibrous carbon has a place in cleanroom filter media. “We have looked at combining both granular and fibrous carbon materials into one filter media. It would be our granular-based material in a pleated format, followed by a layer of these activated carbon fibers for a media that would provide extremely high efficiencies and last longer than conventional media,” says Middlebrooks.
The cost of such a product would be even higher than the already expensive carbon-loaded media. It won't be available until cleanroom users justify an investment in that level of performance, a decision that depends on improving the sensitivity of AMC monitoring.
Combination AMC media
While activated carbons are a good overall means of AMC control, they are not necessarily the best solution for specific applications. To that end, media makers are looking at additional functional particles. Ion-exchange resins and other modified polymers are being used for the removal of ammonia and other amines. Combinations of adsorbents can also be used to achieve multiple AMC removal with the same filter.
AQF has teamed with Purafil Inc. (Dorville, GA), a chemical-media and filtration-systems manufacturer, to combine AQF's particle-loaded non-woven process with the Purafil Select activated alumina technology. The alumina is impregnated with potassium permanganate, a highly reactive material effective for the removal of corrosive, odorous and toxic gases.
AQF is the contract manufacturer for the resulting Purafilter-branded chemical filtration products, which include the CPS-500 media for outside air and general odor control, and the PSAM-500 for control of both acids and bases. “Recirculated air inside the fab is not technically segregated from one process area to another. Amines, ammonia and acids from one process area can be redistributed to another. The PSAM-500 allows users to address a wide range of contaminants in a single filter,” says Chris Muller, technical services manager at Purafil.
Plus, both media are available with an integral particle air filter for cleanrooms without additional space for the installation of chemical air-filtration systems within their air-handling units. These combination particle/chemical filters replace existing low-efficiency (30-35 percent) pre-filters and high-efficiency (85-95 percent) final filters in fan filter units and distributed fan systems using the existing air-handling system. Muller reports that 80 percent of the Purafilters sold are the combination filters using PSAM-500 chemical media and an 85 percent particle media.
AQF also has investigated combining its carbon-loaded media with a HEPA-type particle media. “Users would get two-in-one, but the concept's chief limitation is that after layering the two media together, the pleats aren't as tight and that creates undesirable pressure drops,” says Middlebrooks. “We don't know yet if it is feasible, but it might be practical for minienvironments with self-contained air-handling systems.”
Smaller line widths and the shift from aluminum to copper interconnects, which conduct electricity better but are sensitive to acid contamination, will make AMC control an ever higher priority for semiconductor facilities in the days ahead. Chipmakers aren't the only users concerned with atomic-sized contamination.
New AMC filter users include biotech facilities, such as in-vitro fertilization labs where fugitive gases can impact the success rate of embryo creation.
New user groups and new processes will continue to spur media innovation. The best that media and filter manufacturers can do is hang on for the ride.