Cleanroom Gloves Grasp Market Attention
Glove manufacturers respond to the rapidly growing cleanroom industry with new materials, new features specifically developed for controlled environments, and a new-found respect.
By Sheila Galatowitsch
For years, cleanroom gloves were a sideline business for glovemakers, whose primary business was manufacturing medical gloves. In fact, cleanroom gloves were nothing more than medical gloves laundered, tested for contaminants and packaged for controlled environments.
Today, however, things are different. As cleanroom glove sales continue to grow, the cleanroom industry is starting to carry more weight with glovemakers, who see an opportunity to profit from materials and features specifically developed for the controlled environment user. The result is a big win for users, who will have a cleaner, more consistent product and a wider variety of materials to choose from.
“People in the glove world are starting to recognize the control industry as a growth area,” says Jeff Welker, director of global marketing for Ansell Edmont`s (Atlanta, GA) new nitrile glove. “Everybody`s trying to get into it and meet the needs of the cleanroom industry.”
The worldwide market for cleanroom gloves is expected to grow from $428 million in 1995 to $808 million in 2000, according to market researcher Robert W. McIlvaine, president of The McIlvaine Co. (Northbrook, IL). Asia is the world`s biggest glove consumer, with orders of $204 million in 1995 and $419 million projected for 2000. North and South America rank second, with orders of $124 million in 1995 and $215 million projected for 2000.
Not surprisingly, the semiconductor, disk drive and pharmaceutical industries use the most gloves, but even the food industry is expected to grow from $16 million in glove orders in 1995 to $26 million by 2000, and flat panels from $8 million to $39 million (see chart). Many of these industries spend hundreds of dollars a year per worker on gloves alone.
Most of the gloves that end up in cleanrooms today are manufactured in Southeast Asia in a labor-intensive process.
To make natural rubber latex gloves, hand-shaped porcelain molds are first coated with powder such as cornstarch or calcium carbonate, then dipped into liquefied latex. Powder is used to prevent the material from binding to the mold. The glove is dipped into the powder again to coat both the inside and outside of the glove. These dipping operations are performed in large areas ranging from open windows with no screens, to more controlled environments.
After curing, the glove is stripped from the mold. Gloves bound for the cleanroom market must be washed in chlorine to remove the powder and prevent the material from sticking to itself, then rinsed in DI water to remove the chlorine. Drying and packaging are the final steps in the process.
No powder is used to make polyvinyl chloride (PVC) gloves, because that material strips differently from the mold. Molds are either dipped in liquefied vinyl in a process similar to natural rubber latex, or the liquid is sprayed onto the mold. No washing or rinsing is required, although vinyl can be processed further with air to remove contamination.
With the cleanroom user in mind, manufacturers are looking to improve the latex glove-making process either by removing the powder while the glove is still on the line or by manufacturing without powder; using a single chlorine wash to replace a double wash and using on-line chlorine gas to replace the liquid chlorine bath. These improvements are still experimental, but in the meantime, manufacturers can perform additional rinsing and inspection steps to make the final product as clean as the state of the art will allow.
After its latex gloves are chlorinated and rinsed with DI water, CT International (San Luis Obispo, CA) transfers the gloves to a cleanroom facility for additional DI washing, a manual inspection and packaging. In January, the company opened a new cleanroom in Malaysia which has more capacity and larger equipment, says Robbi Rizzo, general manager for the scientific division, as well as two 400 ft2 cleanrooms for processing and packaging. The cleanrooms are currently operating at better than Class 100, and the company is working toward Class 10.
When it opened its original facility five years ago, CT International was one of the first manufacturers to process gloves in a cleanroom. But “users keep upping the ante,” Rizzo says, forcing manufacturers to continuously improve their process. “The user environments are getting cleaner and cleaner, so we have to make improvements to get the gloves even cleaner.”
Latex is still the best-selling glove for cleanroom use. Vinyl is a strong second, and the newcomer, nitrile, is quickly taking its place as one of the top three glove materials used in cleanrooms today.
Latex for fit, feel and comfort
Latex gloves are made from natural rubber trees grown primarily in Southeast Asia. The natural rubber is form-fitting, inherently stretchable, and provides the best dexterity of the three. That makes it the glove of choice for working with fine parts or in wet environments.
Baxter International (McGaw Park, IL) has dominated both the medical and cleanroom glove market for over a decade, controlling more than 50 percent of the thin-wall (3 to 7 mil) glove market. Alan Myers, director of marketing for industrial products, says that for the past couple of years, Baxter has seen its competitors from the medical industry pour into the cleanroom market as they observe its dynamic growth.
Baxter`s widely used CR-100 latex glove was developed in the 1980s and continues to be a hot seller. Although the glove has been continuously improved as its semiconductor customers move into sub-micron geometries, Baxter has recently invested heavily in the glove, totally changing the way it looks and performs.
The company has replaced its approximately 3,000 ft2 Class 100 manufacturing facility in the U.S. with two Class 10 facilities in Malaysia. The certified plants, totaling 5,000 ft2, were constructed to Class 1 standards and brought on-line last year. They have ULPA filtration specs that meet the requirements of an 8-in. wafer fab, Myers says. The latex dipping is performed outside of the new cleanrooms in a Class 100,000 medical device facility. Once the gloves are dipped, they enter the Class 10 environments and are subjected to Baxter`s proprietary cleaning and treating process.
The CR-100 gloves that end up in cleanrooms start out as cleanroom gloves, not medical gloves. Baxter developed a dedicated process to make cleanroom gloves about five years ago. “We started doing, acting and thinking differently in the last five years,” Myers says. “We recognized that the needs of cleanroom customers are separate and distinct from the needs of medical customers.”
As popular as latex is with cleanroom users, the material has some significant problems. The most well-known is the serious allergic reactions many people have to the material`s proteins. CT International puts its latex gloves through a warm water leaching process after drying them to remove proteins, and Safeskin Corp. (San Diego, CA) has had success with its hypoallergenic latex glove line.
But latex also falls short in static dissipation–a concern to the disk drive industry where electrostatic discharge (ESD) is a big problem. There are antistatic latex gloves on the market, but they command a premium price, and some users have stayed away from them because of additives. Moreover, as natural rubber becomes worn, it sheds its outer layer, causing particulates; as it ages, it continues to decay and flake. These contamination issues are why some people consider vinyl cleaner than latex.
Clean, cheap vinyl
Because vinyl is a synthetic material, its glove manufacturing process is cleaner than that of latex. As an inert material, it also doesn`t shed. Vinyl is abrasion- and acid-resistant, nonallergenic and has some antistatic properties. It is mostly suited for use in dry environments. Vinyl is also a few cents cheaper than latex, although prices are starting to even out at about 20 to 25 cents for a 12-in. ambidextrous pair. Oak Technical (Stow, OH) invented vinyl gloves in the late 1950s for the medical industry and is now one of the leading suppliers to the cleanroom market.
Latex sensitivity is one reason for vinyl`s recent popularity, says Richard A. Renehan, vice president of Renco Corp. (Manchester, MA). Renehan says the cleanroom user has adopted vinyl as an inexpensive and reliable alternative to latex, which can be subject to natural rubber shortages and seasonal price fluctuations.
Renco imports and distributes the KM vinyl glove from Korea, which is stripped in a Class 10 cleanroom and packaged in a Class 1 minienvironment. The KM glove comes off the assembly line clean, ready for a Class 10 cleanroom. It requires no further processing. “They are packaged and ready for the end user. That`s what keeps the price down,” Renehan says.
Some manufacturers take their vinyl gloves to contract facilities for additional cleaning and final packaging. American Purification & Sterilization Inc. (Ronkonkoma, NY) processes both latex and vinyl in its certified 2,500 ft2 Class 100 cleanroom, but vinyl makes up the bulk of its business, according to vice president Paul Bunce. Newly manufactured gloves come into the cleanroom through a loadlock system, where they are subjected to a blowing stream of HEPA- filtered air, which removes contaminants. The gloves are then packaged in a certified-clean, double-polybag configuration.
Vinyl`s biggest drawback is its loose fit, making it unsuitable for applications which require a fine sense of touch. Sweat from hands can also permeate the material, and fingernails can puncture it. In addition, while latex comes in hand-specific and textured versions, vinyl is sold primarily in an ambidextrous, smooth finish. Adding the former features to vinyl would not improve its fit, but would increase its cost. The shortcomings of both latex and vinyl are inducing manufacturers to investigate new materials, such as the recently introduced nitrile product.
Antistatic, puncture-resistant nitrile
Nitrile is a synthetic rubber polymer that`s been around since World War II, when it was developed as a substitute for natural rubber latex. It resists chemicals and splash better than natural rubber, is naturally static-dissipative, and is three times more puncture-resistant than either latex or vinyl and of comparable thickness. Heavy-gauge nitrile gloves have been used for many years in non-controlled, industrial applications, but it was only six years ago that manufacturers began to consider the material for the cleanroom market.
In 1990, Best Manufacturing Co. (Menlo, GA), one of the world`s largest industrial glove manufacturers, introduced N-DEX, the first thin-wall nitrile glove. The company had hoped it would appeal to the cleanroom user, “but at the time the ionic extractables and particulate levels were higher than required for the critical controlled environments, and the only way to reduce ionic extractables was to change the formulation,” says Marketing Director Logan Boss. Outside the cleanroom market, the glove was a success.
In 1993, Ansell Edmont introduced a nitrile glove clean enough for the controlled-environment user. The company has about 10 percent of the total cleanroom glove market and has led the emerging nitrile market since the introduction of its “Nitrilite” ambidextrous glove. Nitrilite was “developed specifically for this industry,” Welker says, “from the choice of the polymer itself to the formulation and process steps. Everything was totally geared to making a glove for the cleanroom industry–and that`s a first,” he says. During development of the new glove, engineers looked for a clean, durable polymer that wouldn`t break down over time. They also wanted dexterity –gloves that could be thin but strong. They chose nitrile over more exotic materials with similar qualities because it was five to six times cheaper. The Nitrilite glove still needs improvement, Welker says. “We need to make it more like natural latex, more form-fitting.”
Ansell Edmont developed specialized equipment to make the glove by a process similar to that used in the manufacture of natural rubber latex gloves. Cleanroom users adopted the new glove so quickly the company was forced to put its major users on a strict allocation program while it hurriedly tooled up. Ansell Edmont expects to open a new facility in July, which will allow the allocation program to be disbanded.
While its cleanroom glove sales are dominated by latex, the company wants to position the Nitrilite glove as its number one seller for multiple cleanroom applications. After the new facility is brought on-line and the allocation program ended, Welker says Ansell Edmont plans to introduce a softer and cleaner glove every eight to nine months. “We want to develop new products at a rapid rate and put our current products out of business,” Welker says.
Several manufacturers offer nitrile gloves, and others will soon be joining the market. In April, CT International introduced a nitrile glove line, and in the last six months, Best has modified its N-DEX formulation so much the glove may now be clean enough for the controlled user.
How much of the market nitrile will capture from latex and vinyl is uncertain, but some manufacturers predict it could replace anywhere from 30 to 60 percent of the gloves currently used. For that to happen, cleanliness and cost issues will have to be resolved, however. Today, nitrile costs about 20 percent more than either latex or vinyl, and some users still don`t consider it clean enough for certain applications.
Gloves made of specialty materials also have a share of the market. Although these gloves can be pricey–some more than a dollar per pair–many companies see their high performance as worth the added cost. Typically ultra thin, clean and strong, they are used in small volumes for sensitive applications where latex and vinyl might not stand up. Unlike latex and vinyl, which must be thrown away after one use, many of these gloves can be washed and reused, increasing their cost effectiveness.
W.L. Gore & Associates Inc. (Elkton, MD) offers a PTFE-based glove with its Gore-Tex garment suite. Users like the Gore-Tex glove because it is cleaner than latex or vinyl, and because it breathes, allowing hands to stay dry, says Gore`s Hudson Benson. The inert, non-allergenic PTFE glove is used primarily in the semiconductor industry, Bensen says, and was designed from the beginning for the cleanroom user.
Polygenex (Cary, NC) developed its glove five years ago from a proprietary polyurethane formula, a hypoallergenic material used for implantable medical devices and made by a radio frequency welding process. Available in 1.2 and 3 mil thicknesses, they have twice the strength of a typical 6 to 8 mil glove, says Joseph D. McGarry, executive vice president. He says sales of his polyurethane glove– priced at 70 cents per pair– are growing as more cleanroom users discover it.
Growth of glove liners
In addition to the growth of glove sales, suppliers are seeing an increased demand for glove liners. Glove liner sales are up in large part because of allergic reaction to latex. The liners–typically made of nylon, cotton or polyester–are worn as protection against skin rashes, to absorb perspiration, and for comfort. Available in half-finger and full-finger versions, liners can be washed and reused many times. Reusability is key, since liners are costly–$1 to $1.50 a pair. The more a liner can be washed and reused, the cheaper it becomes.
Polygenex carries a line of 100 percent nylon glove liners, available in 8-gram and 5.5-gram weights, and packaged and double-polybagged in a Class 100 cleanroom. The liner can be laundered up to 50 times and can also be autoclaved. McGarry says the company`s glove liner sales in 1995 were double its sales for 1994. “The business is big,” he says, attributable to the fact that gloves can be worn for up to four hours at a time, because the liners contribute significantly to comfort.
Berkshire Corp. (Great Barrington, MA) has been manufacturing a 100 percent double-knit, continuous filament polyester liner since 1988, leveraging its expertise in nonwoven and textile wipers to introduce the liner. Market acceptance has been strong and steady, says Bill Phair, market development manager. “As a manufacturer, we try to listen to the market, and when they ask for something slightly different, we try to be proactive.” In response to user requests, the company has manufactured custom nylon and cotton polyester-blend liners and plans to introduce the new materials to the market at a future date.
Even though liners are worn underneath a glove, there are still potential contamination problems. Spun threads, and edges that are not hemmed or surged, can become loose and cause particulation. Washing and reuse can exacerbate this problem. Knit gloves have a loose thread or “tail” from the last stitch, which should be backstitched. Continuous-filament threads are best suited for cleanroom use.
Polyvinyl alcohol (PVA)–used in the company`s line of cleanroom cleaning products–is the material for a new glove liner from Micronova Manufacturing Inc. (Torrance, CA). Unlike most glove liners, it is not worn underneath a glove but as a slip-on device measuring about four inches long with a hole at one end. The user slips the hole over the thumb and slides the rest of the liner under the glove. The idea behind the liner, called a MicroWick, is to break the seal that forms when latex gloves are worn without a liner, says Micronova research and design engineer Lito De Guzman. “If you have a latex glove on without any liner, a seal forms around the wrist and hand, trapping moisture,” De Guzman explains. The MicroWick breaks the seal and provides a layer between the palm of the hand and the glove, absorbing moisture and moving it to the wrist area where it evaporates. “The problem with an ordinary glove liner is that once it gets soaked, it stays soaked, and the hand gets warmer.” Pricing on the product hasn`t yet been finalized, but it will probably cost 50 cents to one dollar a pair. The liner can be laundered and reused 20 to 30 times.
Testing for cleanliness
Getting a new glove–or any glove–past a quality control engineer is not easy, as many suppliers discover. Most end-users will test a product internally with more rigorous specs than manufacturers could ever imagine. Before they even attempt to interest a company in a glove, many manufacturers routinely use third-party labs for particulate and extractable testing.
Particulate contamination usually arises from the manufacturing process, and test results can help manufacturers trace the source of contamination. Extractability testing looks for traces of elements that come out of the glove when it is placed in certain solvents–elements that may prove harmful in cleanroom applications, such as calcium, chloride, sodium, silicon, sulfites and zinc. Tests are based on the recommended practices (RP) of the Institute of Environmental Sciences (IES). The RP for gloves is IES-RP-CC-005-87-T, known as RP-5.
Particle Measurement Technology (Ventura, CA) is a third-party lab which has been testing cleanroom consumables for 13 years. Martin L. Grossman, director of laboratory services, says he performs two particulate tests on gloves per RP-5: a biaxial shake test that counts and sizes both generated and readily releasable particulates, and a new, near-zero mechanical stress test for readily releasable particulates. The company also performs extractability testing.
More than 20 labs test gloves, using different methodologies and instrumentation, so results may vary from lab to lab. IES recommends that testing be performed at the same lab with the same instrumentation. “There are so many variables in testing,” Grossman says, “but no matter which variables are being employed, the product with the lowest level of particulates will always come out as the product with the lowest levels.”
Latex gloves have grown cleaner through the years, Grossman says, while vinyl`s particulate level has been consistent. “The major players in nitrile all seem to be at the same place,” Grossman says, “but there are efforts to come up with nitrile low in particulates and extractables.” Polyurethane gloves tend to stick together in packaging, and pulling them apart generates particulates. Overall, latex has the lowest level of particulates in the tests he has performed, Grossman says, but hastens to add, cleanliness levels on individuals brands will vary.
Whiskering and scaling
While RP-5 serves as the foundation for glove testing, most end-users will develop their own testing protocols for specific applications. Motorola`s (Mesa, AZ) Supplier Quality Engineer Ken Epstein goes beyond RP-5 testing in order to qualify materials for Motorola`s Logic and Analog Technologies Group. “The testing maturity on cleanroom consumables is very young,” Epstein says. “We are three to four years away from comprehensive testing on consumables.”
Epstein developed his own testing methodology about four years ago when the company had trouble finding a suitably clean latex glove. Using magnification and special lighting, Epstein can identify surface conditions he calls “whiskering” and “scaling,” which are forms of scuffing. These terms signify surface anomalies occurring during the manufacturing process when latex sticks to the molds. Scuffing creates potentially releasable particles on the surface of a glove that cannot be eliminated by washing or cleaning. Epstein performs a simple test that involves rubbing gloved hands together to see what particles are freed or generated on the surface. With faulty gloves, “you are quickly going to get to a point where friction will heat the surface and break down the brittle latex at the points of scuffing,” says Epstein, who has trained manufacturers in the protocol and helped develop ways to stop the anomalies from occurring.
Motorola`s glove qualification process takes a minimum of three months to perform. After Epstein prescreens a glove, the manufacturer is required to have it tested at Motorola`s designated test lab for particles and extractables. Depending on those results, the glove will go through three to four internal fabs for testing. The final stage is a wear test for fit, feel and comfort. After passing all requirements, the glove may be considered for use by participating fabs at Motorola.
Even though glove testing must still be improved, Epstein says he is seeing cleaner gloves today than four years ago. What is needed now are longer gloves, he says, 15-in. lengths, in addition to the 10.5- and 12-in. lengths currently available. Some manufacturers, in fact, are now developing the longer lengths.
Double-gloving at Rockwell
Line workers at Rockwell Semiconductor Systems (Newport Beach, CA) actually wear two gloves in the company`s die manufacturing cleanroom–a Class 100 vinyl glove over a nitrile glove. Despite cost and comfort drawbacks, the company`s yields are significantly higher with the double-gloving procedure, says Pat Warton, senior technical staff member responsible for quality assurance.
The nitrile glove is donned first and worn throughout the gowning process, and the vinyl glove is donned before the workers enter the cleanroom. The procedure was implemented after the company had problems with fingernails punching through the vinyl gloves. Nitrile`s puncture-resistance helped solve that problem, but the gloves couldn`t be worn alone because Rockwell`s tests showed elevated levels of sulfur in the glove. “By donning the PVC overglove, the sulfur and other ionic species are contained, so there is no contact transfer,” Warton says.
Rockwell`s tests show vinyl to be cleaner than any other material submitted for evaluation, including polyurethane, nitrile and latex, and, adds Warton, the glove is less expensive. Workers who complain about hand fatigue or heat, or individuals with allergic reactions to nitrile are given a cotton polyester blend glove liner to wear in addition to the two gloves.
Double-gloving is not common in the semiconductor manufacturing industry and is not likely to become a trend because of the cost prohibitions, Warton says. But until the perfect glove appears on the market, Rockwell is committed to the procedure to maintain its quality and yields. Warton thinks glove manufacturers need to improve their manufacturing process, treating gloves more like medical devices and less like commodities. They also need to investigate new materials–specifically composites.
Manufacturers are doing just that. “Everyone in our marketplace is evaluating the next alternative to latex,” says Baxter`s Myers. “We continue to explore all options relative to the next generation. It may be nitrile or a synthetic material that doesn`t exist today.”
“We are looking at what is going to be the next material–looking at either making it cleaner or with better ESD qualities, or a better fit, feel and comfort,” says Jerry Van Horn, president of Omni Sales Corp. (Denver, CO), a glove manufacturer and distributor. “We are on the cusp of some big changes as far as what`s going to be used in cleanrooms.”
The market may never produce a single glove suitable for all applications, but cleanroom users can be sure that their influence with glove manufacturers will continue to grow–along with sales of cleanroom gloves.n
Baxter International`s CR100 non-sterile, latex, hand-specific glove which is packaged under Class M3.5 laminar flow hoods using bag-in-a-bag packaging.
One of CT International`s cleanroom gloves that is processed in a cleanroom and has packaging specifically designed for controlled environments.
The worldwide glove market is expected to almost double by the year 2000, according to the McIlvaine Co. (Northbrook, IL). The biggest glove user is Asia which isn`t too surprising due to all the semiconductor fab and disk drive manufacturing
on that continent.
Ansell Edmont`s Nitrilite gloves were developed especially for controlled environments. “Everything was totally geared to making a glove for the cleanroom industry,” says the company`s Director of Global Marketing Jeff Welker.
A view inside CT International`s packing room at its newest cleanroom. built in Malaysia. As user environments get cleaner, so to must gloves, according to CT`s Robbi Rizzo.
Berkshire Corp.`s BCR glove liners are made with 100 percent continuous filament polyester and are constructed using a special seamless technique to prevent gloves from raveling.
Beneath the glove, slipped on around the thumb, is the newest glove liner from Micronova Manufacturing, Inc. The MicroWick whisks moisture away from the hand to evaporate outside the glove. The glove liner can be laundered and used again.
Classifying Contaminants in Cleanroom Gloves
By Dr. Sebastian Plamthottam
Choosing the right glove depends on a number of factors such as cleanliness, ergonomics and cost. Besides those factors, glove customers must wade through the wide selection of materials–natural rubber (NR), vinyl (PVC), nitrile rubber (NBR), Teflon (PTFE), and urethanes–as well as features–hand specific, ambidextrous, long length, etc. In addition, cleanliness requirements vary according to the application and the industry. For example, a disk drive manufacturer may be more concerned with trace levels of corrosion-inducing contaminants than would a semiconductor or pharmaceutical manufacturer. Also, within each industry, the requirements may vary, depending on process technology and the cleanliness requirement of the product and the environment. Cleanliness specifications will be more stringent in certain stages of disk drive or semiconductor manufacturing or processes than in others.
The most common types of contaminants in cleanroom gloves are particles, ions, non-volatile residues (also known as molecular contaminants or NVRs) and certain elements. Transfer of these contaminants can occur through touch, liquid-borne or airborne contact.
The type, size and quantity of particles on the surface will vary depending on the type of glove–dust or environmental particles from the material of the glove–for example, rubber hydrocarbon, residual powders from glove processing, salt residues, and certain rubber insoluble residues or bloom. The number and size distribution of particles can be characterized by liquid or airborne particle counters using IES-RP-005-87T procedures or by optical microscopy. Particle type or composition can be characterized by a number of techniques, including Scanning Electron Microscopy-Energy Dispersive X-ray (SEM-EDX), Fourier Transform Infrared (FTIR) Microscopy, and Electron Spectroscopy for Chemical Analysis (ESCA).
Ions. Commonly known as ion extractables, these entities comprise anions such as chlorides, sulfates, nitrates etc., or cations such as sodium, potassium, calcium, zinc, magnesium, titanium or aluminum. The type, amount and mobility characteristics of the ion can influence yield in certain industries. Certain mobile ions (Group IA in the Periodic Table of Elements) such as sodium can induce conduction and low field breakdown in semiconductor devices. As circuit geometries get thinner, risk of failure increases. Certain cations such as chlorides can induce corrosion, a significant factor in disk drive (thin film) manufacturing. Glove selection should take into consideration analysis and characterization of these contaminant residues to maximize yield and reliability. Commonly used ion determination methods are Ion Chromatography (IC), Inductively Coupled Plasma (ICP) and Atomic Absorption Spectroscopy (AAS), such as Flame-Atomic Absorption (AA) or Graphite Furnace-Atomic Absorption (GF-AA).
Nonvolatile Residues. NVRs are residues from evaporation of a volatile liquid that can outgas at high temperatures or a high vacuum of space. Gravimetric and spectroscopic methods can be used to quantify and identify NVRs in gloves, which arise from molecular contaminants such as plasticizers, surfactants, waxes, silicone fluids and oils. PVC gloves generally contain high levels of NVR. Controlling these contaminants is critical in the semiconductor, disk drive, aerospace and optical industries. In the disk drive industry, NVRs can cause stiction and headcrashes. In space optics, NVRs, when subjected to the temperature and high vacuum of space, can vaporize and recondense on optical devices, resulting in attenuation of signals by absorption, scattering, diffraction and interference fringes. NVRs can also cause adhesion failures in coatings, paints and adhesives used in the aerospace industry. In semiconductor manufacturing, NVRs can produce mechanical interference in mask precession and in microlithography.
Elemental. Certain elements–such as Boron, Aluminum, and Phosphorous–are critically controlled in Polysilicon. As contaminants, they can act as unwanted dopants. Elements can arise from particles, ions or extractables. These contaminants needs to be evaluated in areas where they are considered critical to the process and the product. Inductively Coupled Plasma-Mass Spectroscopy (ICP-MS), Graphite Furnace-Atomic Absorption (GF-AA) are analytical tools to determine the presence of these elements.
Standards and testing
There are two standards generally used to specify cleanliness. Fed-Std-209E specifies the cleanliness standards for the environment, and Mil-Std-1246B specifies cleanliness standards for a product.
Fed-Std-209E: “Airborne Particulate Cleanliness Classes in Cleanrooms and Clean Zones.” This standard specifies air cleanliness classes for cleanrooms by specifying the maximum allowable number of particles per cubic meter/foot of air and their distribution relative to size.
Mil-Std-1246B: “Product Cleanliness Levels and Contamination Control Program.” This specification standardizes cleanliness levels for both particulate and NVRs on surfaces, assemblies, liquids and gases that are subject to contamination control.
Cleanroom glove testing
A number of tests evaluate and specify the cleanliness of gloves used in controlled environments. One widely accepted method, recommended by the Institute of Environmental Sciences for cleanroom gloves and fingercots, is IES-RP-CC-005-87T, which provides procedures for testing and evaluating cleanroom gloves. This includes tests for particulates and extractables, and recommended testing standards for permeability, physical properties, accelerated aging, chemical resistance, static dissipation, barrier integrity, heat resistance, microorganisms and corrosion.
Cleanrooms in the semiconductor industry generally operate at Fed-Std-209E Class 100 to Class 1. As geometries for semiconductor devices continue to shrink and packaging densities increase, particulate contamination of less than 0.5 microns on gloves will become more critical.
Cleanroom gloves should have low airborne and liquid-borne particle contamination, acting as an effective barrier against human hand contamination. Any standardized testing for particulate should include the maximum possible value representing the hi ghest possible risk (i.e., worst-case testing.). The IES-RP-005 87T method determines the quantity and size distribution of particles on the inside and outside of the glove surface utilizing a liquid-borne determination method. The result is reported in particles /unit area of the surface. A very stringent test that provides a high level of particle release, it is normally more consistent and reproducible than airborne particle tests–when test procedures and critical test variables are carefully controlled. However, it does not characterize particle type. If testing with a laser particle counter, the numbers are not absolute, but they are useful in monitoring the performance of cleanroom gloves vis a vis particle contamination. Since laser particle counting is an indirect method, bubbles, large particles, and contaminants on the glove could interfere with the result and should be closely controlled.
In comparing liquid particle counts between test laboratories, it is important to ensure that test conditions and measuring equipment are comparable. Particle counts depend on a number of test parameters: (1) the amount of water used for extraction (2) time and method of agitation (ultrasonic, orbit shaker, bi-axial shaker, etc.) (3) the amount of surfactant (4) method of particle counting–automatic vs. microscopic (5) particle size ranges, and (6) type of equipment. Excessive agitation–for example, ultrasonics–could generate particles not originally present as free particles on the glove surface.
In the Helmke drum airborne dry test, results are strongly influenced by test conditions and the static and adhesion properties of the glove surface. This method does not release as many particles as the liquid test, and is not recommended if maximum possible contamination levels are needed to demonstrate maximum risk, especially if the glove is used under wet conditions.
This test is intended to identify and quantify the total amount of inorganic ions (positive and negative) present on the glove surface. An ion chromatograph is used to determine anions and cations after extraction, as described in ASTM D 4327-88. For elemental determination, Inductively Coupled Plasma (ICP), Flame-AA or GF-AA may be used. Normally, ion extractable determinations are reproducible and useful in specifying cleanliness. For increased detection limit, GF-AA or ICP-MS are used.
It is important to note that this is a stringent test. In actual end-use application, ion transfer from the glove under dry transfer conditions will be substantially lower than the values obtained in this test. To minimize ion contamination, gloves should be selected on the basis of low extractables levels and good barrier properties for human perspiration and skin oils (i.e., lowest possible pinhole levels, consistency of barrier integrity).
This is commonly determined by gravimetric methods with a suitable solvent or by spectroscopic procedures after extraction. The results are normally reported in mg/glove, mg/g of glove or mg/sq.cm. of glove surface. Solvent extractable levels are a function of the nature of the glove material and the type of solvent. It is important to note that PVC gloves contain large amounts of plasticizer (20-40 percent) and tend to give very high levels of extractable NVR. They are not recommended if NVR contamination or outgassing is a critical concern.
Electrical and mechanical properties
Most organizations follow standards set by the EOS/ESD Association (EOS/ESD S-11.11-1993), EIA, ASTM, DOD, FTMS and NFPA. Normally, cleanroom gloves are tested for surface resistivity, volume resistivity and static dissipation where static properties are critical. Nitrile, vinyl and urethane gloves generally tend to outperform natural rubber, unless the rubber is specially compounded for static dissipation.
Mechanical properties are measured to determine the strength of the glove material and are usually run in accordance with ASTM D412. Tensile strength is the maximum tensile stress at breakpoint expressed as lbs/in.2 or MPa. Elongation is the maximum tensile strain developed in a test specimen at break in a tensile test (i.e., maximum stretch before break) and is expressed in percentages. Tear strength is the force required to tear a specimen of given thickness.
In addition to the described tests for cleanroom gloves, depending upon specific end-user applications, tests for permeability and diffusion, chemical resistance, heat resistance, bioburden, barrier integrity and corrosion are also conducted.
In comparing test results, it is important to ensure that the test conditions and the units used to report results are comparable and the test method reproducible. n
Dr. Sebastian Plamthottam is president of Sebastian Scientific Services (Upland, CA; phone (909) 946-8896), an information, testing and R&D services company which he started in 1995. He completed his Ph.D in Polymer Science from the University of Akron in 1980. Prior to starting Sebastian Scientific Services, he worked for two Fortune 500 companies in polymer R&D. He currently consults with many R&D organizations, and is a consultant to Absolute Quality Leadership Inc. in the development of new cleanroom glove products.