Cleaning the cleanroom
Cleanroom wipers and swabs are important to enhancing productivity and product quality in critical manufacturing environments.
By Bruce Flickinger
New technologies, chemistries and value-adds notwithstanding, cleanroom consumables are driven by two tenets for suppliers and users alike: They have to function properly and consistently, and they have to do so cost-effectively. For wipers and swabs, this means removing undesirable materials from the cleanroom without leaving undesirable material behind. These tools are critical to keeping environment and product contaminant-free, but their ubiquity and utilitarian demeanor tend to place them low on the list of considerations when a company is re-evaluating its operations for cost or quality reasons.
Where change is warranted, such as where rising defect rates or interruptions in the supply or quality of a particular consumable merit a revisiting of suppliers, performance evaluations are needed to compare products and make informed decisions. But change doesn’t come easily for critical manufacturing industries, especially those that are regulated, and audits, plant expansions, equipment upgrades, and the like usually are perceived as having a greater impact than revisiting the use of wipers and other consumables.
“Users aren’t likely to change their cleanroom procedures or products unless there is significant benefit to doing so,” says Kim MacDougall, research scientist with Kimberly-Clark Professional (Roswell, GA). “It is very difficult to bring about change in wiper usage in cleanrooms because many times consumables are a lower-priority product area.”
“If the time is available, a company will entertain the idea of re-evaluating the performance of their wipers and deciding whether other products should be considered for quality or cost efficiency reasons,” adds Howard Siegerman, PhD, director of technology for ITW Texwipe (Mahwah, NJ). “Users need to weigh the question, ‘If we start this performance evaluation, do we have the resources to carry it out? And what is the benefit in doing so?’ If it ain’t broke, don’t fix it is a prevailing attitude.”
Follow the money
This attitude is changing. Tolerances are becoming tighter across the critical manufacturing spectrum and in hard disk drive (HDD) fabrication, in particular, processing lines and equipment are becoming smaller, making particulate and ionic contamination a necessarily bigger concern. “With shrinking linewidths and 300 mm wafers in the semiconductor market, particle contamination will inherently increase in significance,” says Chris Roman, business manager with Milliken Anticon Products (LaGrange, GA).
In HDD manufacturing, John Westland, president of American CleanStat LLC (ACS; Anaheim, CA) notes that consumable suppliers have had to adopt new materials used for swab handles and different static-dissipative and chemical-resistant additives to foam swabs. “Some manufacturers are offering specific tools and tweezers that are antistatic or made of other inert tip and product materials, and new nitrile and polyurethane materials for gloves and fingercots are both static dissipative,” he says. “There has been a lot of pressure on packaging and wipers to reduce particles and static tribocharging by using ESD [electrostatic discharge] formulations that don’t outgas.”
The issue, Westland says, is that “these industries are high volume and these items are not made in large enough quantities to allow prices that are as low as the traditional non-ESD safe materials.” The situation presents a “push-pull” between cost and performance for HDD manufacturers, who are not only operating on slim profit margins but also face increasing competition as other types of memory devices-for example, optical, flash, and USB-erode the HDD marketplace.
Another related business driver is burgeoning overseas microelectronics manufacturing, fueled by inexpensive labor. Where operations are more labor intensive, wipers and other consumables figure more prominently in the costs-benefits equation.
“The semicon market is growing globally and developing differently than it has in the U.S.,” MacDougall says. “Labor and facilities are driving the biggest migration of jobs, and where operations are labor intensive, the use of consumables increases.” In the U.S., conversely, the trend in semiconductor fabrication is to “engineer out” consumables and the human workers who use them as much as possible.
As such, the consumable testing market has been relatively flat in the U.S. over the past two to three years, observes Carl Newberg, president of Micro-Stat Labs (Rochester, MN). “This can be attributed to a couple of things, namely a large portion of manufacturing moving to Asia; an increase in the numbers of independent laboratories in Asia, which work for a fraction of what labs have to charge in the U.S.; and the downsizing of quality departments. The typical engineer just doesn’t have time to worry about consumables.”
Providing the independent lab’s perspective, Newberg says that most of the major players in the HDD industry are moving to a 0.3 μm cumulative large-particle-count specification for many of the components that go into the drive. “While many of the consumables specifications haven’t changed yet, I expect them to in the near future,” he says. “The suppliers haven’t been hit with these new specifications yet; however, when they are, I would expect the number of qualified suppliers to drop somewhat.”
Suppliers obviously want to provide a high-quality product and avoid a blip in the end user’s productivity or product integrity that could be traced to a poorly functioning wiper. Superior but more costly products that provide lower contamination levels tend to make sense over the long term. “It’s important to consider the ongoing cost of utilization in terms of how quality or productivity is being affected,” says Texwipe’s Siegerman. “Suppliers have to prove savings through cost and use models, in reducing waste and saving money in the overall system.”
Other geographic challenges that weren’t always there are shaping business practices as well. “As off-shore manufacturing becomes more common, auditing becomes less practical. A company could contract a local lab to audit the supplier and conduct testing, but lab quality and the results they generate can vary,” MacDougall says. “Most customers prefer to transfer the burden of testing to their supplier. They typically aren’t equipped to do their own RP [recommended practice] testing.”
“RP testing” refers to recommended practices developed by the Institute of Environmental Science and Technology (IEST), the U.S. standards-setting body for scientists and businesses concerned with contamination control. The analytical techniques used to detect ionic, particulate, and other contaminants in cleanroom wipers and swabs are specified in IEST RP CC004; this document’s current iteration is familiarly known as RP 4.3. First published in 2004, this RP describes methods for evaluating, selecting, and testing wipers used in cleanrooms and other controlled environments for characteristics related to both cleanliness and function. It is applicable to wipers, for the most part, although a companion swab RP document is wending its way through the same working group.
Test and measurement criteria addressed in RP 4.3 include particles (MM/m2), extractables (%), ions (ppm), fiber shedding (MM/m2), and composition. Specified analytical techniques include ion chromatography, liquid particle counting, or scanning electron microscopy; nonvolatile residues (NVRs) are to be assessed using gravimetric weight analysis or, where organic contaminants need to be identified, by Fourier-transform infrared spectroscopy or gas chromatography combined with mass spectrometry. These are sensitive, highly evolved techniques that not every manufacturing concern is equipped to conduct.
“There are also ASTM methods [American Society for Testing and Materials, recently renamed ASTM International] for general testing that are not specific to the lower levels that are of interest to many HDD and semiconductor cleanroom manufacturers,” Westland adds. The Electrostatic Discharge Association (ESD), for one, provides test methodologies to measure electrical properties such as surface and volume resistance, which directly affect static charge build-up and discharge voltage. Many companies also specify testing that addresses their own cleanroom personnel or operational needs.
RP 4.3 is due for review this month, but this effort had not moved forward as of this writing and details were unavailable from the RP 4.3 Working Group. Westland believes proposed changes will focus primarily on test method specifics, the amount of material to be tested, and the agitation or movement methods for particles. A standard test has been added for use with alcohol or acetone, he says, to show test results with solvents other than water. The working group also aims to standardize the particle sizes to test, types of equipment to use, and calibrations of blank samples and bracketed ranges for tests to be relevant and comparable. “Each lab interprets the test methods slightly differently and this creates differences among various labs, which can change results dramatically at 0.5 μm and smaller,” Westland says.
Seeking standardized testing
Variable test methods and results are cited as the foremost obstacle to objective wiper evaluations and comparisons. Siegerman expresses a common opinion that “the same testing methods are not being used, and individual facilities have their favored approaches to given tests, although we are reaching better commonality in our testing practices.” RP 4.3 is regarded as a meaningful and practical means of achieving standardization.
Most end users of cleanroom wipers are well aware of the IEST test methods, says Milliken’s Roman, and although not all wiper manufacturers follow these methodologies, IEST and ASTM methods “allow fair comparisons of wiper attributes and cleanliness profiles.”
The optimal way to assess performance of wipers and swabs is through a set of independent tests or through an evaluation of use and yields on finished products. But these “are not always totally independent tests,” Westland notes. “It is best for users to either run the tests themselves or have the tests run by an independent lab.” End users also can ask the manufacturer to run a current lot of material for tests and specify which lab will be used to do the comparative tests.
While highly sophisticated, leading-edge technology companies conduct their own consumables performance testing, many end users rely on their suppliers to do the work and provide the necessary documented quality assurances. Suppliers, in turn, conduct this testing in-house and, in the optimal scenario, invest in a third-party lab for independent corroboration. “The major manufacturers and major end users of cleanroom wipers perform their own in-house evaluations to ensure ongoing consistent quality with identical operators and test equipment,” says Bill Lynch, vice president of sales and marketing with Lymtech Scientific (Chicopee, MA), which enlists third-party test labs to verify test data.
“Users in the markets we serve have specific cleanroom wiper characteristic needs and requirements,” Lynch says. “Some of these specifications include qualification documentation regarding sterility validation, lot controls, product and process compatibility, along with independent third-party performance test data of ultra-low extractables, particle counts, and bioburden levels.”
Where products are being compared, the only way to do this fairly and accurately is to test all the products in the same manner using the same test methods, with the same test labs or departments doing the tests. “You cannot always correlate or rely on different manufacturers’ test specifications or results because they use different methods, labs, blanks and units of measure for testing all the variables that can affect sub-micron particle testing, outgassing, NVRs, or other parameters,” Westland says.
“It is important for end users to understand that it is very difficult to compare wipers with a side-by-side review of the manufacturers’ specification sheets,” Lynch adds.
Testing is used to quantify a number of product attributes, but two broad performance criteria are of critical interest: the particle and fiber shedding characteristics of the wiper, and its sorbent rate and capacity. These respectively correlate to the wiper’s proclivity for leaving potential contaminants behind, and its aptitude for picking contaminants up. This information is critical not only for the end user but also gives the wiper manufacturer information about the consistency of its process.
Cleanroom wipers are used in close proximity to the end user’s products, sometimes even wiping the product itself, as is the case with medical device and some microelectronics manufacturing. Here, Siegerman notes, “The needs of medical devices are changing because devices such as stents are beginning to incorporate active pharmaceutical components.” Swabs, too, while commonly associated with environmental surface sampling, also contact product and are used for cleaning tight geometries in equipment and fabrication tools, such as slots, O-rings, and recesses in vacuum ports. A swab also can be used to capture particles by lightly touching the back of a wafer.
While essential requirements are the same, specific needs diverge somewhat between life science and microelectronics applications. “In cleaning a pharmaceutical aseptic fill area, wipers are used as applicators for disinfecting and sanitizing solutions. You need to dispense large volumes of liquid across broad surfaces,” Siegerman says. “The wiper cannot contaminate the application process and has to be chemically resistant. The usual fabric choice here is polyester knit, which satisfies both of these needs: It’s adsorbent, resists most aggressive chemicals, and will not contaminate surfaces.” Wipers dampened with DI water or isopropyl alcohol solution are also used to remove solutions and disinfectant residues from surfaces.
Users in microelectronics and semiconductor manufacturing also tend to favor polyester knits, but in these environments the emphasis is on removing particles. Dampened-as opposed to saturated-wipers are the optimal means for doing so. Observers make the general note that it is important that wipers are not moistened to the point of saturation because if 100% of a wiper’s capacity is utilized, then there is no capacity to retain excess liquid and contaminants. “Pre-moistened wipers should be engineered to deliver a specific amount of cleaning solution and then to re-adsorb that solution along with any contaminants,” Roman says.
One point to note, explains Siegerman, “is that the ability to wipe a surface dry has nothing to do with the absorbency of the wiper; it depends on the mechanism of water pick-up.” Synthetic knit wipers that are surface-treated to pick up water adsorb the liquid, holding the liquid in the interstices of the knit, or on the surface of the polyester strands. Cellulosic wipers absorb water into the fibers of the wiper, causing them to swell. Wipers made from cellulosic materials offer better absorbency but typically aren’t found in clean environments better than ISO 7.
Although laundered polyester knits are used in as much as 90% of electronics and life science applications, the material’s main drawback is that polyester holds moisture via adsorption, which entails treatment with a surfactant during the manufacturing process. This process can leave residues and result in inconsistent sorbency among lots.
Newberg affirms that some synthetic products that meet end-user requirements for ISO 5 or ISO 3 and better actually “don’t hold moisture very well and, in some cases, don’t do an optimal job of picking up debris, either. The dry synthetics also don’t have the ESD properties that some end users would like. This forces them to make decisions about their use in ESD-safe work areas, and how they perform their wiping and cleaning operations.”
Advances are being made constantly in materials and their treatments to improve particle retention and reduce ESD. One example is Milliken’s Particle Attraction Technology, or P.A.T., which involves proprietary chemical modification of the polyester yarn to impart increased sorbency and particle attraction within the fabric. The technology is used in the company’s line of Anticon wipers.
Figure 4. Texwipe’s Vectra Honeycomb 10 is a sealed-border polyester wiper designed for grit and residue removal in ISO Class 3-4 environments. Photo courtesy of ITW Texwipe.
Unlike many suppliers of cleanroom wipers, Milliken makes its own base fabrics and thus can readily incorporate the P.A.T. treatment into its manufacturing process.
The company uses computer control to run its manufacturing process, from yarn texturing and knitting to finishing, wiper dimensioning, and cleanroom laundering. Statistical process control (SPC) is prevalent throughout all of the textile processes in the company’s facilities, as is the case for most wiper and swab manufacturers. “We provide lot test data to our customers and also do significant upstream process testing to ensure our processes are in control and that final results will be within specifications,” Roman says.
Similarly, at Lymtech, Lynch says, “SPC is a fundamental key component to ensure our manufacturing process is consistently and verifiably in control. Our end users are extremely interested in statistical process control, quality protocols, and process testing. Some of our testing includes dosimeter measuring of irradiation dosages, independent third-party sterility validation testing, and liquid particle counts, to name a few.”
End-user audits are also part and parcel of conducting business in cleanroom wipers, Lynch says. “The common practice is for a team of two or three quality auditors to review the entire cleanroom wiper manufacturing and packaging process. These auditors verify that we are strictly adhering to our stated SOPs, including use of the necessary forms, labels, tags, and other documentation.” Additionally, “some end users will have special requirements-for example, testing of specific ions that might be important to their people, product or process,” adds Lynch.
Working with wipers
While the emphasis of these efforts is on optimizing a wiper’s absorbency and affinity for offending particulates, the flip side of a wiper’s ability to attract and retain particles is that they become “very effective traps for airborne molecular contaminants,” Siegerman cautions. “It is critical to keep them bagged. Don’t leave them sitting out.”
Figure 5. Kimberly-Clark’s Kimtech Pure dispenser for pre-saturated wipers allows one-handed dispensing with no need to hold a package or reseal a closing flap. Photo courtesy of Kimberly-Clark.
Users need to be mindful of this and other logistics of cleanroom wipers-their storage, cutting, or other preparation, unpacking, movement in and out of areas of different levels of cleanliness, ultimate disposal, and the like. Procedures governing all of these activities need to be in place.
Human variability in wiper use cannot be overlooked. “Some cleanroom personnel are very advanced in the use and control of wipers; they follow strict protocols for opening a package and bringing wipers into a clean environment,” says KC’s MacDougall. “Folding is important where you don’t double-use a side, and I’ve seen folding techniques just short of origami.” But other technicians are less skilled in the vagaries of wiper use, and “there’s always opportunity for improvement.”
One new option on this front is the “Kimtech Pure” dispenser for pre-saturated alcohol wipers, which allows one-handed dispensing with no need to hold a package or reseal a closing flap. The unit addresses two issues faced when dispensing wipers: “It is tough to pull out just one wiper, especially with a gloved hand, and packages can be difficult to reseal, causing pre-saturated wipers to dry out,” MacDougall says. “There is a definite perception of the potential for contamination with an open package of wipers.”
For additional cost efficiencies, some users repurpose used wipers from the cleanroom for use in less critical applications, such as maintenance. “We see a handful of end users who use a regrading system whereby the cleanroom wiper is used for its intended critical process, then the used wiper is relaundered and reused in a much less critical area,” Lynch says. “The relaundered wipers are packaged entirely differently than the original high-purity wipers. Regrading represents a contamination control risk factor in the sense that the relaundered wiper has lost all of its original purity and cleanliness and must never be confused with a high-purity wiper.”
From materials and testing to handling and use, there are numerous considerations when evaluating the cleanliness and effectiveness of new swab and wiper products, a task that ultimately falls upon the end user. This, in turn, entails evaluating the supplier and its proficiency at producing a quality, cost-effective product. And while wipers and swabs might not be foremost items on a company’s list of operational concerns or opportunities for change, they are critical contributors to the cleanliness of the manufacturing environment.
Resources and contacts
American CleanStat LLC
1596 S Anaheim Blvd., Bldg B
Anaheim, CA 92805
300B Route 17 South
Mahwah, NJ 07430
1400 Holcomb Bridge Rd.
Roswell, GA 30076
60 Depot St.
Chicopee, MA 01014
River’s Edge Technical Service
3612 3rd Pl. NW
Rochester, MN 55901
Milliken & Co.
201 Lukken Industrial Dr. W
LaGrange, GA 30240