The overlooked contaminator–bacteria
To the Editor:
In theory, cleanrooms get regular maintenance, however, those of us in the business know better. Once a cleanroom area is up and running–will the managers shut it down for maintenance? This is not likely unless, for example, the ceiling tiles are falling down.
There has been extensive attention to dust, metal fragments, gases, chemical species in the cleanroom literature but little mention of bacteria. The earliest mention I know of is cited by Don Tolliver [2], and the effect of biologicals was unclear. This is a little surprising because it was already known that many bacteria and viruses were transmitted through the air [3]. Some have believed that HEPA filters “hold back” the bacteria from entering the cleanroom. Hospital workers find that idea very amusing. In fact, the bacteria deposit on HEPA filters grow through the filter and enter the room (within 3 weeks to 6 months). One report on just this effect was done at IBM Burlington, VT [4]. Once the filters were changed, bacteria downstream was reduced to zero. However, after three weeks cleanroom workers reported bacteria from the HEPA filters on the wafers.
Indeed, the problem begins in the central air conditioning units of the building, where there are always “wet spots.” It is my experience and that of IBM`s that these wet spots are “alive.” More data of this kind may be found in Reference 5. Most of the data available on bacterial contamination was reported in the 1980s; isn`t there anything new? I raised that question with a medical industry worker and his reply was, “we all know the bacteria are there and where they come from, what we don`t have is a good removal system.”
Bacteria-killing filters have been advertised. They have been coated with some sort of germicide, but they are ineffective. Most the filter`s effectiveness is from its collection of predeposited dust, while bacteria would never come in contact with the filter itself. It is well known that a clean, new filter is inefficient at dust collection.
Some data taken at the ceiling level of a cleanroom by the author indicates some of the samples were taken on apparatus, others were taken on the “downstream” side of the HEPA filter. The hospital people that “read” the slides commented that, in many cases, there were so many layers of bacteria, they could not separate them. There seems to be no question that the bacteria come through the HEPA filter and deposit on the manufacturing equipment.
We might ask if this source of contamination is so prevalent in cleanrooms, why haven`t there been reports in our industry`s literature? This is a good point. I suggest that engineers, and even cleanroom contamination control personnel, are not trained to recognize bacteria. Bacteria may be mistaken for dust but this is rather dangerous. Bacteria grows rapidly thereby increasing the problem.
Finally, what can be done to see and kill bacteria on filters without actually going to the filter, taking it apart and sampling? I am working with a University Optical Sciences group on an optical system that could be used to scan filters from a distance. One application might be looking at cleanroom bunny suits. We have only performed limited studies, but it appears that cleanroom garments are quickly contaminated by bacteria. The question is, “Does the bacteria get off the garment and onto the devices?”
References
1. Waring, Tony, “Creating a Life-Cycle Cleanroom Maintenance Plan,” CleanRooms, Oct. 1996, pp 37-45.
2. Tolliver, Donald M., “Domestic and International Issues In Contamination Control Technologies,” Microcontamination, Feb. 1988, pp. 18-22.
3. Morey, Philip R., “Experience on the Contribution of Structure to Environmental Pollution” The Center for Indoor Air Research, 1099 Winterperson Rd., Suite 280, Linthicum, MD 21090, Ms. Lynn Kosak-Channing, Tel (410) 684-3777, Fax (410) 684-3729, is an excellent source in the indoor air pollution area.
4. Davies, Carol M. et al, “HEPA Filters as a Contamination Source,” Journal of Environmental Science, Mar./Apr. 1981, pp. 27-35.
5. Morris, Robert H., Indoor Air Pollution:, Heating/Piping/AirConditioning, Feb.1986, pp. 59-68.
Dr. Stuart A. Hoenig, P.E.
Professor Emeritus and Adjunct Professor
Dept. of Electrical and Computer Engineering,
Dept. of Agricultural and Biosystems Engineering
University of Arizona (Tucson)
Particle counter update
To the Editor:
I read with interest the article about particle counters (“Detecting Contamination in Semiconductor Processing is Key for Particle Counters,” CleanRooms, Nov. 1996,䂍). It provides a valid overview of the market, applications and technology for contamination monitoring. However, this leaves several questions open which I would like to bring to your attention and maybe I can provide an answer to them.
The article completely neglects to mention products of the second largest particle counter manufacturer in the world, Met One Inc. (Grants Pass, OR). Although, Terra Universal is mentioned as a supplier of particle counters, it is Met One, Inc. who manufactures all particle counters Terra Universal offers.
The list of particle counter sources identified by CleanRooms should include the following international companies: Rion (the third largest manufacturer of particle counters in the world) Hitachi, Kennomax, and Sibata, which are all in Japan. Next, A3, Sfp, Pamas, and Grimm, all in Germany.
Rion has a representative in the United States for liquid particle counters and buys certain products under an OEM agreement from Met One. Sibata sells private-label Met One Particle Counters. Kennomax represents TSI in Japan and manufactures its own particle counter products. TSI sold its particle counter product line to Particle Measuring Systems and has a OEM agreement with PMS to supply CNC to PMS. A3 and SfP originated from Kartel in Germany. Grimm used to distribute Met One in Germany and got into the particle counter manufacturing business. Pamas is closely related to the German rep of PMS, PMT.
One U.S. company that was not mentioned is High Yield Technologies (HYT; Sunnyvale, CA). HYT is the third company in the Pacific Scientific Instrument Division and manufacturers in situ sensor monitoring particles in semiconductor process tools.
Dr. -Ing. Holger T. Sommer
Merlin, OR