Cleanroom Subfloors Harbor an Arsenal of Dust, Vermin and Other Contaminants
By Susan English
San Diego, CA — Cleanroom cleaning service companies find surprising things under the floors of the cleanrooms they service — everything from wads of dust to the occasional fab worker caught napping. This gross contamination can actually work its way up from the subfloor into the cleanroom itself.
Dust tends to be thickest around air returns, which can cause airflow to get stuck, bounce off the dust and create turbulence under the floor. The turbulence can then literally push contamination back up through the perforations. From there, it`s not hard to make the subsequent loss-of-yield/throughput connection. “A cleanroom is not just clean by design; it`s clean by control,” says David MacIntyre, regional manager for Atlanta-based TEC International, a specialist in the full-service cleaning of cleanrooms and controlled environments.
After completing a gross clean on a 1,800-ft2 facility, complete with gowning room and bootie bins, TEC International`s MacIntyre recently pulled almost 9 lbs of dirt from underneath the floor, which hadn`t been cleaned in five years, he says. (A “gross clean,” as defined by the Institute of Environmental Sciences` Mil-Hdbk-407, is a “preliminary or rough cleaning to remove scale, rust, metal chips, shop dirt, etc.; this cleaning is done in a normal work area to visual inspection standards.” When applied to cleaning a cleanroom, it has been defined as “initial removal of gross contaminants such as those resulting from manufacturing processes” and as a necessary prelude to performing an effective precision cleaning.)
Every week, TEC removes three pounds of the dust just from gowning and bootie room floors. Out of the service support areas, including chases and the bootie bin and gowning room, MacIntyre`s company cleans about 6.3 lbs of dirt a month, tracked in by four shifts of about 100 employees per shift. The 9-lb cache of dust taken from the subfloor came from the pedestal tops. “As we vacuum those, we took the floor tiles out. I don`t think they`d ever been cleaned. We averaged about four ounces of dirt per floor tile.” He explains that dust collects in the support channels of the tile and in the louvers that adjust the direction and amount of airflow. In the process aisles — approximately 13,000 ft2– about 12 lbs of debris was collected from underneath the floor, including everything from booties to wipers.
Why the neglect of such key areas in the cleanroom? MacIntyre says it`s a cost issue for today`s fabs. While his company specializes in the cleaning of critical environments, a lot of fabs hire regular janitorial services, which he says just don`t have the knowledge or training to clean today`s high tech installations.
But sometimes, such neglect implies just laziness or lack of knowledge, says MacIntyre. He likens it to using a portable HEPA vacuum in a process aisle. He says companies “spend all kinds of money” to put house vacuums in, then don`t utilize them. “They`ll let their janitorial people take in HEPA-powered vacuum cleaners, which defeats the whole purpose of the airflow,” he says. “Yes, they have a HEPA filter, but what you`re trying to achieve is brush filtration of the carbon dust on the HEPAs. But if the motor is exhausting into the atmosphere, you`re blowing all that carbon dust around,” he points out. Besides the ubiquitous 2-in. wads of dust which are the scourge of cleanrooms everywhere, fabs in the North and Northeast may have rust, hydraulic fluids or the occasional chipmunk sitting under flooring, while in the South and Southwest a variety of critters such as ants, bats, rodents, tarantulas, and even snakes, may crawl through holes left unsealed by subcontractors. Depending on the region, any one of the aforementioned has been found at one time or another by Anthony Hilliard, owner of CleanRoom Cleaning Specialists (CRCS), a division of Micron Consulting Inc., (La Mesa, CA) specializing in cleanroom janitorial and maintenance services on a quarterly, semi-annual and/or annual basis. With offices in San Diego,CA Scottsdale, AZ and Fort Erie, ON, the company offers a “Cleanroom Technicians Course” for training cleanroom management and personnel. Hilliard has advised companies that they could improve their yields from about 85 percent to 95 percent with just an annual cleaning. “But they just don`t want to change anything,” he says.
When called by a company to clean, CRCS personnel follow the Institute of Environmental Sciences Recommended Practices, as illustrated in videos produced by Micron Video International Inc. (Greensboro, NC). They perform an evaluation, one or more gross cleans, as needed, followed by precision cleaning. CRCS`s procedure for gross cleaning starts with the ceiling, then the walls, flat surfaces, and last of all, the floors — all levels. Precision cleaning follows, identified by three categories: Red zone areas designate areas where equipment that is sensitive to the environment is located; Yellow zone areas indicate areas that are not as sensitive to the cleanroom environment, but which need special attention; White zone areas are those areas where cleanrom components are stored both in the cleanroom area and in personnel work areas.
After the initial evaluation, the worker will pull up floor tiles at the return air area, the center of the room and finally, at the far end of the room, where most of the contamination or wildlife tends to congregate.
This helps in detecting the flow of dust particulates, the degree of density in different parts of the room, and any turbulence. The area is then UV-sterilized prior to precision cleaning. Hilliard advises companies to seal up all holes drilled underneath cleanroom flooring with concrete or foam, which contains a natural poison wildlife can`t chew through. n