Cleanroom garments and laundries work to eliminate particulates

By Susan English-Seaton

In the early days of cleanroom garments and laundries, prior to solid state electronics, the initial need for contamination control stemmed from the launching of the first manned rocket into space — the Atlas space program, which raised concerns about sensitivity to contamination from hydrocarbon, which would ignite when combined with liquid oxygen, according to Dick Buechsenschuetz, vice president of Alltex Uniform Rental Service in Manchester, NH.

During the 1960s, Buechsenschuetz was employed at Prudential Overall Supply, then located in Los Angeles, which built its first cleanroom in 1962. At the time, he says, the effort was to define two sources of contamination on garments: particles and fibers. The first testing programs used black light, taking advantage of the fact that many organic materials emit fluorescence, enabling the detection of skin flakes on garments.

“In 1960, we were trying to figure out how to process garments. We started filtering the air and the water to some degree, but it wasn`t really until 1962, when we built a cleanroom laundry at Prudential. As far as I know, that was the first cleanroom laundry in the world. It was built around a dry-cleaning process that Prudential utilized for many years,” Buechsenschuetz says. Garments were processed in a dry-cleaning machine which had to be loaded from the front. The garments were taken out only after the area had been purged of contamination. At that point, he says, there were no standards and no particle counters. “We took a probe and a filter and drew air through the fabric. When the particles became entrained on the filter; we counted them. But again, there was no standard that said `x-count is good and y-count is bad,`” he adds.

In the search for a faster and better control process, Buechsenschuetz commissioned a manufacturer of stainless steel to build a platform. Around its perimeters was stretched a stainless steel screen measuring one square foot across, on which a one-square-foot sample of fabric could be secured. “We could sample using this probe, and for the one- cubic-foot-a-minute flow, we could sample over the top of one square foot of a defined area, and then be able to get close to quantifying how clean things really were.” The new method, of course, was the forerunner of the ASTM F-51 Alternate Method, set forth in the Institute of Environmental Sciences` Recommended Practice RP-CC-003-87-T, which was officially issued in October 1987.

Originally, static electricity was mostly an annoyance, not the destructive force as we know it today in the manufacture of semiconductors. Attempts were made to construct fabric that would conduct without generating a charge, causing it to bleed off instead. One of the early trials was conducted with a polyester fabric called Dacura. Two parallel stripes of rayon fiber were woven into the warp of the fabric. Cellulose was chosen on the theory that because, like cotton, it would absorb moisture, it therefore would conduct electricity. The problem with that, says Buechsenschuetz, was that the cellulose began to break down and fall off. “We were finding too much particulate and too many fibers and couldn`t get the counts down. And it didn`t really work.” The next attempt was material woven with stainless steel fibers in a random pattern. Obviously, those materials would conduct and dissipate the tribocharging throughout the fabric, decreasing the voltage of the potential discharge.

The metallic material was originally developed in the mid- to early 1980s for use in the medical industry, when ether was still being used in operating rooms. A surgeon could generate an electrical charge of between 2,500 and 3,000 volts, which would produce an arc, which could ignite the ether and cause an explosion. With the stainless steel fabric, the charge would be dissipated throughout the garment, keeping the charge below 2,000 volts — not high enough to arc over. Of course, the use of metallic fabric was not at all appropriate for use in semiconductor manufacturing.

In the early days of the semiconductor industry, Buechsenschuetz says, “when a lot of wet chemistry was used,” nylon yarns would dissolve when exposed to an acid vapor. Any carbon, of course, would then be released as a contaminant. “It would dissolve all of the threads of the fabric, and it would just turn pure white! Nylon will degrade in acid; polyester won`t.” Today, most conductive fibers still use carbon as a conductor but encase it in polyester, which is generally impervious to degradation in acid. The nylon filament is extruded through a spinerette, a tiny hole behind which liquid nylon is pressurized and squeezed out like toothpaste out of a tube. The filaments are then chilled so they can be configured into a solid form — nylon yarn. The yarn is driven through an acid bath to soften the surface of the nylon, then run through a carbon slurry, which bonds itself to the softened and dissolved nylon. The slurry and the acid are rinsed off to neutralize the acid. The carbon-coated yarn is then woven into a conductive fabric.