Roger W. Welker, R.W. Welker Associates; and Peter G. Lehman, Ansell Healthcare
Cleanroom gloves are critical to high-technology manufacturing. A brief search of the recent literature on contamination and electrostatic discharge (ESD) shows a surprising lack of published research. One study examined alcohol extracts using infrared spectroscopy for organic species and optical emission spectroscopy for trace element cations. Auger electron spectroscopy was also performed on glove fingerprints. This study showed that a high quantity of plasticizer can be extracted from polyvinyl chloride gloves. Another study dealt with both functional and quantitative laboratory evaluations of gloves. Yet another study demonstrated the effectiveness of aqueous isopropyl alcohol (IPA), pure deionized (DI) water, and DI water plus 0.5% of a surfactant for removing loose particles from the surface of gloves. A correlation between the amount of extractable contamination and the amount of contamination that could be transferred by contact was found in another study.
Considering that gloves are among the most expensive cleanroom consumables, more published work would have been expected. A 1994 study illustrated at least one company's cost estimate, as shown in Table I. In addition to their high relative cost, gloves are among the most likely sources of contamination, primarily because of contact transfer.
Many different types of gloves are used in cleanrooms. Dipped film gloves are one of the most popular types, because they provide a continuous barrier protecting the cleanroom environment and products from contamination by the wearer. Other types of cleanroom gloves include knitted and sewn fabric gloves, some of which are also available with barrier films. Other gloves are intended for specialty applications, such as chemical safety gloves or gloves to provide personal protection from heat or cold. The discussion in this article will be limited by dipped film gloves intended for product protection from contamination or ESD.
Many different parameters can be considered when selecting a glove. Among these are mechanical properties, such as length and thickness, the absence of pinholes, and puncture and wear-and-tear resistance. Contamination considerations fall into two broad categories: functional and nonfunctional tests. Examples of functional contamination tests are contact and near-contact stain, while nonfunctional tests include extractable particles, anions, cations, viable organisms, and organic contaminants. In some applications, functional and nonfunctional tests for ESD properties may also be important. This article concentrates on functional and nonfunctional contamination and ESD tests appropriate for the qualification of cleanroom gloves. Future articles in this series will explore in detail the contamination and ESD behavior of cleanroom gloves in real-world conditions.
Functional Contamination Tests: Contact and Near-Contact Stain
As withmany materials used in cleanrooms, gloves or their extraction products sometimes come in contact with, or are in close proximity to, chips, disks, or other products being manufactured. Two types of tests can be used to evaluate the functional suitability of glove materials for cleanroom applications in these two circumstances-contact and near-contact stain. Other tests may be specified, depending on the users' functional requirements.
In a contact stain test, an apparatus to hold the test material and product is prepared in such a way that the apparatus's contribution to the test is negligible. Several strips of the material being tested are held against the product. The apparatus is then sealed within a polyethylene plastic bag to prevent gases from adjacent bags that contain test specimens from interacting. The bags are then placed in a temperature/relative humidity (TRH) chamber for conditioning. Many different companies use this test. Typical conditions are 70 to 80 C and 70 to 85% RH for a period of 4 to 7 days. At the end of the test, the TRH chamber is returned to ambient temperature and humidity under noncondensing conditions. The product is removed from the chamber and inspected for signs of stains, discoloration, or corrosion. This may be done by unaided eye inspection or inspection using magnification.
The near-contact stain test is virtually identical to the contact test, except that the material being evaluated is held close to, but not in contact with, the product. Testers must make sure the material being tested does not drip or sag onto the product. The test material is placed beneath the product. Spacing between the material and the product ranges from 250 to 1270.
There is one other consideration in both stain tests. The glove material may come in contact with water, IPA, or other chemicals that extract damaging substances from the gloves. If this is the case, extracts obtained by soaking the gloves in appropriate solvents are used as the challenge material in the functional tests, often as dried residues.
Nonfunctional Tests: Objective Laboratory Measurements
Materials qualified under functional tests are then characterized with objective laboratory tests. The results of these tests are then used to specify the desired properties to the supplier. The tests will quantify parameters such as extractable particles, anions, cations, organic, and viable contaminants. Electrostatic charge can be considered a form of contaminant for some applications. These tests are done because glove suppliers seldom have access to the users' end products to conduct either kind of stain tests.
The extractable particles test, which was originally developed for characterization of natural rubber latex gloves, was one of the earliest test methods to be used. But it was soon discovered that 40-kHz ultrasonic extraction was unsuitable since natural rubber latex was extremely sensitive to damage by ultrasonic waves. Ultrasonic extraction has been replaced by the orbital shaker to remove particles. A glove is filled with filtered DI water spiked with approximately 200 ppm by volume of a surfactant and dropped into a beaker containing the same solution. The beaker is oscillated for 10 minutes, then the shaker is turned off, the glove retrieved, and its liquid contents drained back into the beaker.
After 10 minutes of oscillation, considerable air in the form of tiny bubbles can be entrained into the liquid. Liquid particle counters (LPCs) typically count air bubbles as if they are particles. Thus, a procedure had to be developed to degas the resulting suspension. Two different procedures for degassing are available. One uses ultrasonic degassing. The beaker containing the suspension is immersed in an ultrasonic tank. The power to the tank is pulsed on and off rapidly. This procedure is repeated 10 to 20 times until the suspension no longer effervesces. An alternative procedure allows the suspension to stand, undisturbed, for 20 minutes. The 20-minute stand results typically in a 5x to 10x reduction in particle count versus ultrasonic degassing. Following degassing the suspension is counted using an LPC; current practice is to count using a 0.5 um-resolution particle counter.
Extractable Ionic Content
Ironic contamination is usually extracted in DI water with no detergent. In one company's method, a glove is turned inside out, filled with DI water, and placed on a hot plate at 80 C. This is often called an outside-only leach. Another procedure calls for known-surface-area pieces of the glove to be immersed in 80 C water for 1 hour.
Parameters for selecting a glove are length, thickness, and wear-and-tear resistance
This is an inside and outside leach test. Ionic extraction for shorter times (typically 10 minutes) at ambient temperature is called extraction to differentiate it from leaching tests.
Following extraction, samples are analyzed using anion chromatography for anionic species and atomic absorption spectroscopy for cations. Anions of interest generally are chloride, nitrate, and sulfate, although some end-users also specify phosphate. Cations of interest include aluminum, copper, iron, magnesium, silicon, sodium, and zinc.
Other Contamination Tests
Several other contamination tests are available, including nonvolatile residue (NVR), organic extractable, and viable organisms. In the NVR test the glove is washed with a suitable solvent, often IPA, and the solvent is left to evaporate in a preweighed weighing dish. The resulting added mass is reported in milligrams per square foot of surface area. The NVR test is time-consuming and procedurally difficult and occasionally results in gross errors. There is direct linear correlation between the count of cumulative particles >0.5 um per unit area and the NVR results, as Figure 1 illustrates. The strong correlation between NVR results and LPC tests indicates the NVR test may be redundant.
CE Liquid GraphOrganic materials can be extracted from certain types of cleanroom gloves by various organic solvents. Again, IPA, commonly used in cleanrooms, might be a good starting point for extracting organic residues. In other cases, it might be desirable to extract with more aggressive solvents, such as acetone, methylene chloride, or hexane, to enhance recovery of hydrocarbons, soluble oligomers, plasticizers, siloxanes, or other molecules considered undesirable.
After recovery of the soluble material, the samples can be concentrated by evaporation, as in the NVR procedure. However, instead of weighing the concentrate, some of it is analyzed by Fourier transform infrared spectroscopy, or in the case of extremely complex mixtures, gas chromatography with mass spectrometry detection. Many organic compounds are so detrimental to products or processes that the acceptance criterion is "none detected."
Viable contaminants may be detected by contacting the surface of a culture medium or by pipetting a wash from the glove onto the medium. The medium is then incubated to develop colonies of the viable organisms, which can be identified and counted.
Selection of glove material for use in ESD-sensitive applications is critical. Nitrile is widely recognized as a material suitable for use in the manufacture of products with extreme ESD sensitivity. PVC gloves are also static dissipative but are made pliable through the incorporation of plasticizers-the plasticizers also impart static-dissipative properties. Unfortunately, these same plasticizers can interfere with the performance of disk lubricants and under extreme conditions can interfere with film adhesion in plated products. Thus, there may be a limited range of materials available to test for certain applications.
A number of different parameters can be used to specify the ESD performance of cleanroom gloves, including bulk and surface resistively, discharging time, residual charge retention, and tendency to tribocharge. Bulk and surface resistively are classical methods for specifying the conductive properties of materials and are often important in the selection or qualification of materials for use in the static-safe workplace. Discharge time is also critical since the time to arrival at a safe voltage level often determines the material's suitability for use in a given application. Residual charge is especially important in laminated or composite structures, where the continuous-phase material in contact with the external environment can be highly insulative compared with the bulk of the laminate or composite structure.
Of these parameters, the tendency to tribocharge - that is, to acquire or impart a charge when rubbed against or separated from a dissimilar material - is the most controversial. The repeatability and appropriateness of tribocharge testing is so in question that "no one test currently available can predict general tribocharging properties for a specific material."9 Since there is no agreed-upon standard for tribocharge testing of materials, attempting to specify gloves from the standpoint of tribocharge properties is, at best, a difficult prospect.
Thus, the remaining options are to test gloves for their bulk and surface resistively, discharge time, and residual charge retention. Bulk and surface resistively tests are reliable since they are based on accepted test methods. Discharge time tests are useful because they are based on accepted test standards and reflect the expected performance of materials in their intended application. Residual charge retention tests are based largely on experience with packaging materials and are appropriate for gloves made of laminated or composite structures.
Bulk or surface resistively can be measured using a number of different standards. Standards considered particularly appropriate are those of the EOS/ESD Association. A direct correlation can be established between bulk or surface resistively and discharge time. Discharge time is also covered by standard test methods, including Method 404 in Federal Standard 101C.11
Discharge time performance has become an industry norm in the specification of gloves for use in the manufacture of hard disk drives. In order to measure discharge times, an individual holds his or her hand on a 20-pF charge plate. The plate and operator are charged to some starting voltage, and the time to discharge to some starting voltage, and the time to discharge to a target voltage is calculated. The most generous disk-drive discharge requirement is from + 1000 V to <+100 V in under 5 seconds, while the most demanding requirement is for discharging from + 1000 V to <10 V in <500 milliseconds.
The charge on the charged plate is either conductively coupled to the subject, as in the case of a conductive or dissipative material, or it is capacitively coupled, as in the case of a insulative material. When the subject grounds his or her body using the wrist strap, the charge on the body will drop to zero and the charge on the charged plate will drop proportionately. Thus, when testing a natural latex glove, the charge measured on the charged plate will drop when the subject grounds, but the charge on the plate will not drop to zero volts. However, in the case of one charged-plate monitor (NOVX, San Jose), the charge on the plate is measured by draining charge to ground through a 100 G bleed resistor, and current is measured using an electrometer.
Glove Use Strategies
There are many different strategies for the use of gloves and glove liners that influence testing considerations. The choice of glove liners is end-use dependent. Some companies use glove liners as gowning gloves. Employees wear the glove liner as they put on their cleanroom garments and place them in a laundry bin after use, just prior to donning a pair of cleanroom gloves. Some individuals continue to wear the glove liner and wear a pair of cleanroom gloves over them to enter the cleanroom. Industries that require manual dexterity often prefer a half-finger glove liner. In most industries, it is the wearer's prerogative whether to use a glove liner in the cleanroom, and many choose not to wear a glove liner.
All these choices affect the test strategy. Full-finger glove liners made of insulative materials might interfere with the ESD performance of gloves during the use, although half-finger liners made of the same materials may not interfere with the ESD performance of gloves since the fingertips are in contact with the glove material. Finally, a full-finger liner made to be static dissipative may offer some advantages over a full-finger liner made of insulative material.
Initial Qualification Versus Ongoing Lot Certification
During initial glove qualification tests, there are rarely enough resources available to determine the supplier's ability to achieve the desired contamination performance by a controlled process. The initial functional tests and benchmark measurements used to determined quantitatively the levels of contaminants on the gloves are typically done on one or two batches of gloves. The degree with which these initial batches represent the population at large should ideally be checked regularly.
The most cost-effective way to perform this ongoing testing is to choose the lowest-cost test or tests that are likely to detect the greatest variability in the gloves. It is the author's experience that viable contamination is almost always one or fewer colony-forming units per glove. Most suppliers can easily meet their published claims for anions, cations, and organic extractable materials, even if they are not regularly testing for these variables. Conversely, it is the author's observation that particle counts are highly variable on gloves. Since the LPC test is also relatively inexpensive, it seems logical that a preliminary lot-screening plan begin with extractable particles testing, using the LPC method.
As critical dimensions decrease in microelectronics manufacturing, the tolerance for contamination and ESD diminishes as well. Gloves are one of the most critical consumable supplies used in cleanrooms and have a high probability of bringing contamination or ESD to chips, heads, and other high-technology products. In order to overcome these difficulties, two types of tests are needed for the qualification process: functional and objective laboratory tests. Functional tests determine the glove's suitability for its intended use with the product. Objective lab tests may then be used to qualify the product for procurement and establish the basis for ongoing lot certification tests of gloves.
To learn more about chemical safety solutions, contact Ansell & Mallinckrodt Baker at www.AnsellMBI.com
This work was sponsored in part by Ansell Protective Productions.
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Roger Welker is founder and principal scientist of R. W. Welker Associates. He was senior director of application technology for Lighthouse Worldwide Solutions (Milpitas, CA). Before joining LWS, Welker spent 15 years in high-technology development and manufacturing at Mircoplis, Seagate, and IBM. He holds a BS in physical chemistry from the University of Maryland (College Park). Welker has authored or co-authored more than 60 papers and is a member of the Institute of Environmental Sciences and Technology, the American Association for Aerosol Research, the Electrostatic Overstress/Electrostatic Discharge Association, and the Data Storage Institute. (Welker can be reached at 818/368-0557 or firstname.lastname@example.org .)
Peter G. Lehman, PhD, was vice president, R&D, at Ansell Protective Products, Coshocton, Ohio. Before joining Ansell, he worked for 10 years in the pharmaceutical industry in Melbourne, Australia, where he managed the manufacture of pharmaceutical active raw materials and designed and construed cleanrooms. He has a PhD from the University of East Anglia (Norwich, UK) and completed postdoctoral work at the University of Grogingen, the Netherlands, and the research school of chemistry at the Australian National University in Canberra.