Cleanroom filters evolve to provide 99.99999 percent efficiency

Cleanroom filters evolve to provide 99.99999 percent efficiency

By Susan English-Seaton

HEPA (high efficiency particulate air) filters had their beginnings after World War I as a result of the world`s first experience with chemical and biological warfare. They were developed as filter media for gas masks to protect against poisonous mists and dusts. In the late 1930s, “absolute” filters, as they were called, were used as a nuclear fission application in swimming pool reactors to filter out harmful radiation exhaust in the manufacture of the atomic bomb. In the 1950s and early 1960s, as HEPA filters expanded their use from atomic energy plants to cleanrooms, they were originally placed at the air-conditioning unit. Subsequently, they were moved into the ductwork, closer to point-of-use, then packaged as a unit in the ceiling. Ultimately, they have found their place directly above the outlet to the cleanroom.

When one company had the idea of moving the HEPA filters as close to the diffusers as possible, this change, along with a high rate of air change and rigid protocols for personnel, greatly reduced the contamination level of conventional-type cleanrooms. By the time Willis Whitfield had perfected the concept of laminar flow at Sandia Laboratories in 1960, the idea of incorporating a complete wall or ceiling of HEPA filters was born. The first standards for cleanliness were 0.5 micron, and paper was an effective media on small particles.

The new standard — the ULPA filter

Within the last twenty years, a new standard for filters was developed: ultra low penetration air, or the ULPA filter. Used primarily in the semiconductor industry, ULPA filters boosted particle capture size from the HEPA standard — 0.03 microns — to 0.12 micron-size particles. Before fiberglass, asbestos was the filter media of choice in ULPA filters. Efficiencies have since increased with demand for smaller and smaller line etchings on wafers. These have gone down from 0.35 micron to 0.25, and now to below 0.2 to accommodate 0.15 micron line widths fairly soon, predicts Peter Jeanseau, director of field engineering at HEPA Corp. (Anaheim, CA).

In fact, the biggest factor in filter development has been the improvement of efficiencies, says Steven Klocke, director of engineering at Flanders Filters, Inc. (Washington, NC). “When they were first brought out, the maximum level of efficiency attainable with the fibers and media was 99.97 or 99.99. We have progressed to super-ULPA filters that are “five nines” (five nines after the decimal point, or 99.99999 percent). The 99.999 measurement has been available for some ten years; five nines has been available only in the last five years or so,” Klocke explains.

Performance issues

The other critical performance issue is pressure drop. The trend of the last ten years toward lower values for lower energy consumption has culminated in a number of different technologies. These include mini-pleated filters, which provide a typical value on the order of 0.35 inches of water at 90 feet/min for a filter at 99.3 or 99.4 percent efficiency. However, the combination of low pressure drop with proper distribution of air at lower velocities has brought efficiencies to 60 to 70 feet/min, says Klocke. “The original designs were all at 90 to 100 feet/min. They`ve been able to prove over the last ten years that a good cleandown can be provided at 60 to 70 feet/per min.”

The ultimate trend has been to lower the airflow through the filter, which, in turn, lowers the upfront cost of filters, because coarser, less expensive fibers can be used at lower velocities. Obviously, lower airflow means less energy expended to move the air. That also means that both the filter design and filter media can be tailored for the lower velocity. “Today`s products are heavily weighted toward a 3.5- to 4-inch deep filter for a pressure drop in the 3.5 inch range at an efficiency of 99 and 5 or 6 nines (percent). We have the capability to make filters with lower pressure drops by pleating deeper, but that`s an area that has not taken off — again, due to having to maintain uniformity of airflow throughout the cleanroom,” says Klocke.

Other important filter performance properties are cleanliness and outgassing of construction materials and sealants. There is a need for sealants that are very low outgassing, says Klocke, and many users have their own protocols for approving those materials. However, an industry standard for determining appropriate outgassing levels is probably a long way off because of the diversity of applications and processes.


While overall filter efficiency really hasn`t changed that much over the last ten years, says Klocke, more and more emphasis has been concentrated on scan-testing, or leak-testing. “A filter can have a very minor leak and still pass the overall efficiency tests. Therefore, the minor leak test has become much more stringent. We are applying automated scan-testing methods, computers sensing the output, checking for very low levels, and moving the criteria for leaks — routinely 0.01 percent of the upstream challenge — down to 0.001 percent. To perform those tests, it takes technology and an environment that`s as clean as the end-user`s.

“HEPA and ULPA manufacturers are being pushed to produce and provide filters made in a clean environment and bagged in one, so that they arrive at the job site as clean as the room in which they`ll be used, instead of waiting for cleanups, wipedowns and cleandowns as they start the room up,” Klocke says.


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