Getting a Grip on Gloves

To The Editor:

The two articles on cleanroom gloves (“Cleanroo0m Gloves Grasp Market Attention,” May 1996, p. 12; and “Classifying Contaminants in Cleanroom Gloves,” May 1996, p. 20) were excellent and informative. There are several points, however, which I would like to clarify.

In the “Cleanroom Gloves Grasp Market Attention” article:

Most gloves are made on porcelain molds. However, some are still made on metal molds.

Powder use can be avoided by using chemical treatment to prevent the gloves from sticking to the molds.

The chlorination process prevents latex gloves from sticking to each other by forming a thin film of neoprene on the glove surface. Neoprene, in this case, is made by chlorine atoms displacing hydrogen atoms on the latex polymer.

Somehow, users have gotten used to the term, “Made (or packaged or cleaned) in a cleanroom.” Unfortunately, the air in the room can be as clean as the class dictates but if the surface`s “clean” materials come in contact with less than the surface cleanliness level required for that product (reference: Mil-Std-1246), air cleanliness is meaningless!

Ken Epstein`s particle count method is under study by WG005 of the Institute of Environmental Sciences, the working group which writes IES-RP-CC005.2. It may add additional information in two ways:

1. The low-power microscopic examination should allow users to determine the surface roughness of gloves.

2. WG005 has avoided the “Rub the Gloved Hands over the Funnel Connected to the Aerosol Particles Counter” test because: the particle counter was not getting all of the particles, and some were outside the counter`s size range; the repeatability of a tester`s hands over a long period of time was questioned; catching the particles in a liquid could overcome the first objection, and if the number of samples to be tested by one person is limited, the second is also.

WG005 will also consider puncture resistance of gloves.

Regarding the article, “Classifying Contaminants in Cleanroom Gloves,” the references to Mil-Std-1246B and IES-RP-CC005-87-T are out of date. Mil-Std-1246C was issued in 1994 and will be revised by the Institute of Environmental Sciences in the next two years as an ANSI standard. Unbeknownst to the author, at about the time CleanRooms printed the article, IES-RP-CC005.2, “Gloves and Finger Cots Used in Cleanrooms and Other Controlled Environments” was issued. This contains a revised liquid particle extraction procedure and has been reformatted to the ANSI format. It too will be revised within the next two years.

The IES-RP-CC005.2 liquid particle extraction test is designed to remove the maximum number of particles under a given set of parameters, not “the maximum possible values representing the highest possible risk,” as Dr. Plamthottam states. If his criteria were used, one could shake a sample until it disintegrated, and thus obtain the maximum counts. When the current particle test was written, a Round Robin test was run to verify that conditions were both repeatable and maximized. For instance, the shake time was run at 5, 10, and 20 minutes. It was determined that although the particle count increased with shake time, the difference between the 10- and 20-minute counts was negligible. A standard shake time of 10 minutes was established.

Hal Smith

Chairman, Working Group 005

Institute of Environmental Sciences

Maintaining a Conductive Pathway for ESD

To the Editor:

In his comprehensive article (see “Controlling ESD at the Cleanroom Workstation,” July 1996, p. 34), Rob Linke (Director of Marketing, The Texwipe Company) succinctly identifies the essentials of a workstation layout to assure the “dissipation of any generated charges.” The first item of his list calls for “dissipative garments for personnel…” However, it is to be noted that when subsequently describing the use of carbon-filament-core polyester fibers in these garments, he astutely observes that in addition to breaks in the fibers reducing the static dissipative capability of the material, the generation of the carbon particles themselves could “cause serious problems in a controlled cleanroom environment.”

Interestingly enough, and for whatever reason, the antistatic capability of the materials that have carbon yarns either embedded or incorporated into their construction has historically been expressed in terms of surface resistivity. Since these materials are believed to be inhomogeneous, the unsuitability of their resistivity measurements as a point of reference can easily be recognized. If electrodes are placed on the fabric to measure resistivity, contact to the conducting threads can easily give a low value. Such measurements reveal nothing about the surface`s capability, between the conducting threads, to retain a charge–even when the carbon-filament-core fibers are not broken. These fabrics can charge to appreciable voltages by rubbing, and have been found to retain the charge for long periods [1].

The fact of the matter is that this phenomena was brought to the community`s attention over a decade ago [2,3]. Although the use of static protective garments is quite commonplace, they do not have what may be termed as zero static charge. In reality, the textiles from which the garments are made are capable of producing or generating a minimum amount of static and the full potential of the synthetic fibers is contained by the introduction of carbon yarns in various configurations. Nevertheless, even though a person wearing the garment may be grounded by one of a number of methods, i.e., conductive footwear or grounded seating, there are no deliberate steps taken to ground the garment being worn by that person. Noting that antistatic materials are not acceptable where they are relied upon to provide an interconnecting conductive pathway, one can only conclude that whatever static is generated on the garment is suspended on the nonconductive polyester fibers.

Under that set of circumstances, the garment would then act as a Farady Cage by shielding and protecting items from electrical events which occur outside of the cage. However, the algebraic sum of all the contained charges would be inductively coupled to the cage and a corresponding E-Field would emanate from the entire outside of the garment. On the other hand, if the cage or garment was grounded, the entire outer surface would have a net charge of zero with respect to the ground and there would be no field to emanate from the garment.

Thus, if it is one of the intended purposes of a cleanroom antistatic garment to protect external items from any charge existing inside the garment, it would appear that the garment itself should be at ground potential as an additional control measure. The grounding of the garment would thus form a protective Faraday Cage capable of preventing the garment`s internal charges from being inductively coupled to the items outside of the garment. This principle is further supported by Section 6-3.3.2 of the NFPA code that states: “Electrostatic charges can set up dangerous potential differences only separated by materials which are electrically non-conductive. Such insulators act as barriers to the free movement of such charges, preventing the equalization of potential differences.”

A study on testing the efficiency of antistatic garments that was published in the April/May 1992 issue of ESD Technology confirmed the fact that “one of the most critical items to check on material is its ability to ground the finished garment. The most common method of grounding is contact at the wrist by the inside of the material. If the conductive threads do not make contact with the bare skin, the material will not bleed a charge away.” [4]

With regard to the use of static-dissipative apparel, it is to be noted that Section 6.6.2 of the 1993 edition of the Institute of Environmental Sciences (IES) Recommended Practice 022.1–Electrostatic Charge in Cleanrooms and Other Controlled Environments states: “a conductive path should be provided between static-dissipative apparel and the ground to complete the system.” [5]

Based on the experience of their peers, those charged with the responsibility for designing workstations to effectively minimize the problems associated with ESD should perhaps consider implementing this additional preventive measure.

References

1. Chubb, J., “The Essential of ESD Grounding,” Evaluation Engineering, Sept. 1995.

2. Rupe, B.I., “Electrical Properties of Synthetic Garments with Interwoven Networks of Conductive Filaments,” Microcontamination, May 1985.

3. Belkin, N.L., “Antistatic Textiles in ESD Applications,” Evaluation Engineering, Jan. 1986, pp. 52-57.

4. Johnson, F.L. and Steffe, D.D., “Evaluating ESD Smocks,” EOS/ESD Technology, April/May 1992, pp. 20-23.

5. IES-RP-CC022.1: Electrostatic Charge in Cleanrooms and Other Controlled Environments, Institute of Environmental Sciences, Mount Prospect, IL 1992.

Dr. Nathan L. Belkin

Clearwater, FL

Supporting THE USE OF minienvironments

To the Editor:

As a confirmed minienvironment supporter, I must respond to some of the statements on minienvironments (see “Japan–Tackling the Challenges of Today and Tomorrow,” Aug. 1996, p. 12). For the record, I would like to comment with the following:

1) Minienvironments would not be in use today if they were not “suitable for volume manufacturing.” In fact, they are the very best transition from totally manual-operation cleanrooms, which are not inherently productive, and fully-automated, “lights-out” automated fabs, which are typically cost prohibitive in the United States. Taiwan Semiconductor Manufacturing Corp. (TSMC), in Taiwan, is in volume production with minienvironments and is clearly competitive with many foundries and conventional fabs in low-cost, high-volume production.

2) Many studies in the United States and in Europe have shown that materials currently used in pods and minienvironments do not significantly outgas contamination to the wafer against today`s surface specification criteria. The ongoing controversy is: How much organic carrier-based desorbed contamination is acceptable for a full flow leading-edge device process? For the most part we do not know, and cannot readily determine that level of contamination during the rest of the 1990s.

3) Minienvironments provide control of the contamination at, or near, the wafer surface to a much greater extent than conventional cleanrooms. We know from multiple studies in the industry, minienvironments outperform all conventional cleanrooms in particle performance below 0.5 micron in size.

Wafer fabs can now purchase advanced vertical furnaces capable of measuring and controlling the oxygen content of the wafer environment between the built-in stocker and the furnace itself. Nitrogen-purged (low oxygen) load stations, with up to 400 or more wafers, prior to high temperature loading, are not a challenge.

4) Nitrogen-purged tunnels for wafer transport were being discussed in Japan in 1985. Normally, they are too expensive for U.S. manufacturing and no one in the United States uses them for full wafer transport to my knowledge. Minienvironments once again offer a low-cost, practical, volume solution to protecting semiconductor wafers between different pieces of equipment. Wafers spend most of their life outside the process tools and that interim environment needs more and better control over time.

5) By the end of the 1990s, large and small semiconductor producers will be finding cost-effective means to fully use minienvironments in volume manufacturing. In my opinion, Japan needs to join us in this development and help the semiconductor industry meet the worldwide demand for complex chips.

Korea too, will be involved. DRAM manufacturing in Korea has essentially overtaken the competition in Japan. In my opinion, we should expect to see focused attention on the implementation of minienvironments in Korea over the next five years.

I believe we are no longer in the position of questioning the usefulness of minienvironments in semiconductor fabrication. The only remaining questions are: How soon will minienvironments be used? In how many different places around the world?

Don L. Tolliver

Consultant

SC Technologies, LLC (formerly Staticon, Inc.) has an updated brochure on its line of long-term ESD protection and contamination control products for cleanrooms, minienvironment enclosures, automated assembly equipment, and other protected process areas. SC Technologies develops static dissipative plastic materials for sensitive manufacturing environments. Information: SC Technologies, LLC, Greenwood, CO (800) 525-8183…MKS Instruments` HPS Division (Boulder, CO) has expanded its custom vacuum products department. This department can modify standard parts and create totally custom parts, such as manifolds and chambers. The manufacturing area has the capability to manufacture large chambers with flanges up to 36-inches in diameter…BioFit Engineered Seating (Waterville, OH) has added an on-line catalog to its World Wide Web site. To get more information, go to www/bioenseat.com or e-mail: [email protected]…Clean Modules Ltd. (Derbyshire, UK) is building a brand new 16 &#165 6 meter Class 10,000 clean molding room for Owen Mumford, Ltd. (Oxon, UK). The Cotswold Division of Owen Mumford, a medical device manufacturer, needed a new cleanroom at its Chipping Norton site to accommodate five molding machines…JMC Environmental Systems (Concord, MA), a division of J.M. Coull, Inc. has been awarded two contracts by ADE Corp. (Newton, MA). The first project is to design and construct a 5,500-ft2 Class 1,000 cleanroom in ADE`s new facility in Westwood, MA. The second contract is for the construction of 85,000 ft2 of new interior office, manufacturing and cleanroom space, also in Westwood. The building will house ADE Corp.`s headquarters where they build microelectronics process equipment used by semiconductor manufacturers to make silicon chips. In addition, JMC Environmental has been contracted to design and construct a 400- ft2 Class 10 cleanroom for Varian Corp. (Gloucester, MA). Varian Corp. is a manufacturer of ion implanters and will use its new facility for a contro