Roger W. Welker, R.W. Welker Associates; and Peter G. Lehman, Ansell Healthcare
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ESD Considerations
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.
Conclusion
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.
Acknowledgment
This work was sponsored in part by Ansell Protective Productions.
References
- RN Roberts, VA Russell, and GO Ramseyer, "Cleanroom Gloves as a Source of....Contamination," Microcontamination 3, no. 10 (1985):57-60.
- L Hecht, "A Study of Cleanroom Gloves," in Proceedings of the Annual Technical Meeting ....of the Institute of Environmental Sciences, (Mt. Prospect, IL: Institute of Environmental ....Sciences, 1991), 560-562.
- JM Kolyer, "Removing Particles from Cleanroom Gloves with Non-ozone-depleting ....Cleaners," MICRO 13, no. 2 (1995): 38-43.
- AJ Hartzell et al., "Correlating Extraction and Contaminant-Transfer Test Results for ....Cleanroom Gloves," MIRCO 14, no. 10 (1996): 69-79.
- GL Knoth, presentation materials from the IBM contaminnation control course (Paris, Fance, ....April 19-21, 1994).
- "Cleanroom Gloves and Finger Cots," Recommended Practice CC-005-87-T (Mt. Prospect, ....IL: Institute of Environmental Sciences, 1987).
- RW Welker, previously unpublished laboratory data.
- RW Welker, previously unpublished laboratory data.
- D Cooper and R Linke, "ESD: Another Kind of Lethal Contaminant?" Data Storage 4, no. 2 ....(1997): 49.
- Eletrostatic Overstress/Electrostratic Discharge Association Standard S11.11, 1993 (Troy, ....NY: EOS/ESD Association, 1993).
- Federal Standard 101C, Method 404 (Washington, DC: Government Services ....Administration).
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
.)
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.
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