Know what's in your cleanroom wipers

11/01/2004

How much contamination does a wiper create and how well does it wipe? This two-part article arms users with the essential information to make the best wiper choice

By Howard Siegerman

Contamination control is becoming ever more stringent in the semiconductor, data storage, pharmaceutical and medical device industries. This in turn focuses attention on the cleanliness of cleanroom consumables. Not surprisingly, wipers, which are widely used for cleaning critical surfaces, come under increased scrutiny for inherent contamination characteristics. Both the users and the manufacturers of wipers want to know what's in the fabric that might affect its ability to wipe surfaces clean. Note that the information developed for a particular wiping substrate can be directly applied to swabs and mops made of that same material because only the form factor is different.

The Institute for Environmental Science and Technology (IEST; www.iest.org) is one of a number of standards organizations that develops methods for testing cleanroom consumables, and it has recently updated its Recommended Practice¹ on wiper testing to incorporate advancements in testing procedures. The IEST document will be cited where applicable in this paper and referred to as "RP4.3". Wiper testing was reviewed previously by the author², prior to the issuance of the new IEST document.

Wipers used in cleanrooms generally fall into one of three classifications—synthetics, naturals and blends. The synthetics include polyester, nylon, polypropylene, polyurethane foam and polyvinyl acetal (PVA) foam—for example, fabrics or structures that are man-made. The naturals, as the name implies, originate from materials that occur in nature—cotton, cellulose, rayon (processed cellulose), and abaca. Blends are heterogeneous combinations of synthetics and naturals.

On the cleanliness spectrum, synthetics tend to be the best, naturals the worst, and blends in the middle. But what exactly do we mean by cleanliness? Here's where testing can provide some answers, allowing the comparison of contamination levels among wiper classifications (synthetics versus naturals versus blends), within classifications (such as polyester versus nylon in the synthetic group), or between equivalent substrates (such as comparing laundered polyester knit wipers from two different manufacturers).

Before reviewing specific details, it is useful to separate wiper tests into two broad categories—contamination characteristics and physical characteristics. Contamination characteristics deal with the following question: How much contamination might the wiper leave behind on the surface, that is, how much residue can be dissolved from the wiper to deposit on the wiped surface? Physical characteristics of the wiper relate to how well it performs its function, e.g. how much liquid it will hold at saturation, or how strong the fabric is before it tears. Typically, 9-inch x 9-inch (23 cm x 23 cm) wipers are used for testing (although this is not a strict requirement) and test results are reported normalized to area (e.g. per m² of fabric) or weight (e.g. per g of fabric). By measuring the basis weight (areal density) of the fabric, test results can be reported normalized to both area and weight if desired.

Contamination characteristics (arranged alphabetically)

Abrasion resistance—The abrasion resistance of a fabric might logically be considered as a physical characteristic, but, because the abrasion process produces particles, it is better classed as a contamination characteristic.

Although a number of standardized test methods^3-8 for abrasion resistance are available from ASTM International (www.astm.org), they are unsuitable for characterizing cleanroom wipers. The ASTM tests involve contacting the fabric against an abrasive surface, then evaluating the abraded fabric for mass loss, change in breaking load (tensile strength), total destruction of fibers in the abraded area, or physical appearance (highly subjective). These evaluations, however, are of little value in determining whether a wiper could be used to clean up the abrasive residues present in a semiconductor process chamber. For that application, the tendency of the wiper to generate particulate contamination would be of greater interest.

A number of years ago, ITW Texwipe (Mahwah, NJ; www.texwipe.com) developed a unique testing apparatus (see Figure 1) to reproducibly abrade cleanroom wipers and quantify, by air particle counting, the number of particles generated. Details on the method and the equipment have been published.⁹

Figure 1. Abrasion resistance test equipment (air particle counter not shown).Click here to enlarge image

In this test, a 3.8 cm x 5 cm area of the substrate is abraded against a 60x60 stainless-steel mesh under an applied force of 1375 g for a period of one minute at a stroke rate of 120 strokes per minute. Particles generated by abrasion that pass through the screen are collected and introduced into an air particle counter. Particles are counted in ranges from >0.3 µm to >25 µm, with the smaller particle ranges typically exhibiting the higher counts. As expected, wipers made from polyester and nylon knit fabrics display better abrasion resistance than do hydroentangled polyester-cellulose blends. However, there are differences among polyester knit fabrics (see Figure 2). Wiping abrasive surfaces in the machine direction of the knit causes less abrasion of the knitted yarns than wiping in the cross direction.¹⁰

Figure 2. Abrasion resistance comparison of polyester knit wipers.Click here to enlarge image

Electrostatic Discharge (ESD)—The ESD characteristics of wiping materials are not frequently measured, yet ESD can be a serious source of contamination in the semiconductor, flat panel and disk drive and data storage manufacturing industries. ESD characteristics are determined through surface resistivity measurements¹¹ or decay time measurements. Conditioning of the fabric sample in a controlled humidity environment is important for reproducible measurements. Based on surface resistivity measurements, all three groups of wiper substrates can be classed as insulative when dry¹²—i.e. possessing volume resistivities greater than 10¹¹ ohm-cm (equivalent to surface resistivity of 10¹² ohms/square). Based on this information, one should be cautious about using dry wipers near ESD-sensitive devices or hardware. Furthermore, the triboelectric effect created when a dry wiper is used to clean an insulative surface will likely increase the surface charge on the wiper, making it possibly an even greater ESD hazard.

Fortunately, there is a simple solution to this dilemma. Dampening the wiper with any liquid—water, organic solvents etc—reduces the volume resistivity of the wiper to the static dissipative range (between 10⁴ to 10¹¹ ohm-cm or 10⁵ to 10¹² ohms/square surface resistivity).¹³ Surprisingly, it does not matter if the liquid used to dampen the wiper is conductive or not. So, dampened wipers can be safely used to clean surfaces near ESD-sensitive areas. Material on ESD basics, measurements and definitions is available from several sources.^14-16

Fibers—Fibers, defined as discrete contaminants with lengths >= 100 µm and aspect ratios (length/width) of 10:1, generally originate from wiper edges and are a serious source of contamination. For this reason, many wiper manufacturers offer polyester knit wipers with some sort of edge treatment, e.g. thermal, laser or ultrasonic cutting of the wiper to seal loose yarn fragments to the wiper body, or a full border seal around the perimeter of the wiper.

The RP4.3 document contains complete information on testing of wipers for releasable fibers, incorporated within Section 6, Test for Particles. The procedure involves immersing a wiper in one of three liquids—deionized water, 9-percent isopropyl alcohol (IPA) in deionized water or 0.005-percent nonionic surfactant in deionized water—then agitating the wiper and liquid with either biaxial or orbital shaking for five minutes. The tester can select the liquid that most closely approximates the liquid that will be used for cleaning activities. In the test, loose fibers on the wiper will be released into the liquid, and when the liquid is filtered through a membrane filter, the fibers will be trapped on the filter and available for counting using an optical microscope. Obviously, low fiber counts are desirable, implying that the wiper will release few fibers to the wiped surface when dampened with one of the three liquids described above, or with some other wetting agent.

Figure 3. Fibers from a polyester knit wiper.Click here to enlarge image

Figure 3 shows the characteristic "U"-shaped, partial loops of polyester fibers released from a cut-edge polyester knit wiper (without edge treatment). The horizontal and vertical grid lines of the membrane filter are approximately 100 µm in width, making it relatively easy to determine which objects to count. Fiber counts are reported as fibers per m² of fabric. It is worth pointing out that the large volume of liquid (600 ml for biaxial shaking, 500 ml for orbital shaking) and the vigorous agitation used in this test will cause much greater quantities of fibers to be released by the wiper than when the wiper is dampened and used as a cleaning device. The test can be considered a "worst case" scenario.

The RP4.3 procedure for fiber testing is similar to that described in an earlier ASTM document,¹⁷ which utilizes a surfactant solution and orbital shaking for fiber release.

The RP4.3 document also draws on earlier methods^18-20 for collecting, sizing and counting particles.

Ions — Ionic constituents in cleanroom consumables like wipers pose problems for the semiconductor, flat panel and data storage industries, because of their effect on device performance and the possibility of corrosion of metallic constituents within the devices. From a releasable ion perspective, laundered knitted polyester wipers are much cleaner than the naturals or blends; the polyester wipers often exhibit <0.5 ppm for each ion measured (i.e. <0.5 µg of each ionic constituent per gram of wiper sample). The advent of high-sensitivity analytical instrumentation such as ion chromatography (IC), with its multi-ion separation and quantitation capability, permits rapid and straight forward ion analyses at sub-ppm levels on a single extract.²¹ Capillary ion electrophoresis (CIE) can also be used for anion and cation determinations, albeit at lower sensitivity. Cations alone can be determined with graphite furnace atomic absorption spectrometry (GFAAS) or inductively coupled-plasma mass spectrometry (ICP-MS). Of the techniques cited here, IC is the lowest in cost and, because of its versatility and sensitivity, it has become the most popular approach.

Ions are typically determined by extracting the cleanroom wipers with deionized water—either at ambient or elevated temperature (the RP4.3 document accommodates both)—for 15 minutes, then analyzing the extractant solution for anions and cations.

Water-extractable ions determined by IC include the halides fluoride, chloride and bromide and the oxyanions of strong acids such as nitrite, sulfate, nitrate and phosphate. A separate IC scan, using different instrumental conditions, provides the capability to determine the water-extractable cations of the Group IA and Group IIA elements such as lithium, sodium, potassium, magnesium and calcium (see Figure 4). Additionally, it is feasible to determine ammonium during the cation scan. Transition elements cations such as Cu(II), Cr(VI), Fe(II), Fe(III), Co(II), Zn(II) and Ni(II) are not readily released from the wiper matrix with a water extraction. An acid digestion is a more efficient technique, with quantitation of the transition metal cations either by IC,²² GFAAS or ICP-MS.^{22, 23}

Figure 4. The ion chromatography (IC) scan provides an analysis of cations extracted from polyester fabric.Click here to enlarge image

Microbials — Microbials present on non-sterile cleanroom wipers are of great concern in cleanrooms because of their particle burden and, more importantly, because of the biocontamination hazard that they represent in pharmaceutical, biotechnology and medical device manufacturing environments. Some of these microbials, given the proper environmental conditions, will reproduce, causing an increase in contaminant level over time.

RP4.3, in recognition of the growing importance of microbial levels in wipers, includes details on how to test for bioburden. Essentially, the wiper is extracted with a buffered solution in an orbital shaker, the extractant is filtered through a submicron membrane filter, then the filter is placed on an agar plate and cultured at a controlled temperature for a defined period. The colony forming units (CFUs) that form on the culture plate are then counted, the counts corrected for extraction efficiency and the corrected CFUs per wiper reported. This information is critical for any sterilization procedures that may ensue.

Artificial fibers such as polyester or nylon have lower pyrogen levels than naturally occurring fibers, e.g., cellulose. Additional information on bioburden is available in the literature.24-26

Pyrogens (endotoxins)— Pyrogens, a special class of microbials, are substances that can cause fever, shock and even death when introduced into the body. Many pyrogens give off endotoxins,27 which are pyrogens in the cell walls of Gram-negative bacteria. These cell walls can cause pyrogenic reactions even if the Gram-negative bacteria are dead, e.g. after sterilization. Because cleanroom wipers are used to wipe down medical devices such as catheters, implants, endoscopy tubes, pacemakers, etc. which are introduced into the body, it is important that these wipers do not contribute significant amounts of pyrogens to the wiped surfaces. Also, parenteral drug manufacturers, which use sterile wipers for cleaning, are concerned about levels of pyrogens in their manufacturing environments.

Although cleanroom wipers are not considered medical devices, treating them as though they are, from a pyrogen perspective, is useful. Wipers made from knit polyester, polyester-cellulose blend, or a polypropylene cellulose composite, were found to contain less than 20 endotoxin units per wiper.²⁸ This is same degree of stringency that the U.S. Pharmacopeia would assign to medical devices that contact blood. Wipers that incorporate natural materials are likely to have higher levels of microorganisms than those made from synthetics. The only practical way to minimize pyrogen content in wipers is to limit the bioburden on the fabric; by extrapolation, this means handling and processing the wipers in the cleanest possible fashion, prior to packaging them for sale.

Although the RP4.3 document does not include details on how to test for pyrogens, it does reference use of the Limulus Amebocyte Lysate (LAL) assay that is normally used for this test. The LAL test is based on the reaction of endotoxins with the blood of the horseshoe crab.^29–33 A description of the gel-clot test method used for measuring the endotoxin content of wipers is available.³⁴ Kinetic-turbidimetric and chromogenic methods are alternatives to the blood clot method in LAL testing.

Editor's note: Look for part two of this article in the December 2004 issue of CleanRooms, where the author will continue with the discussion of contamination characteristics and highlight key physical parameters of wipers.

Acknowledgements

This article makes frequent reference to the IEST RP4.3 document. It is a pleasure to acknowledge the contributions of the many dedicated individuals on the IEST Working Group on Wipers that produced the RP4.3 document. I would also like to acknowledge the efforts of many at ITW Texwipe—Kathy Miscioscio, Wendy Hollands, John Skoufis, Ram Sivakumar and Allen Spivey—as well as numerous previous co-workers whose work is incorporated here.

Howard Siegerman is director of technology for ITW Texwipe (Mahwah, NJ). Contact him via e-mail at [email protected].

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