Evaluating “ESD-safe” contamination control cleaning materials
Douglas W. Cooper, Ph.D., The Texwipe Company
Materials intermediate between conductors and insulators are “static dissipative” and allow gradual draining of charge to ground. “Static dissipative” has been defined by the Electrostatic Discharge Association (Rome, NY) as having a surface resistivity of 105 to 1012 ohm/ square or having a volume resistivity of 104 to 1011 ohm-cm (ESD Association, 1994). Such static dissipative materials are often used in cleanroom floors, tables, packaging, storage vessels, and in the materials that clean them or as mats on which to work or as packaging materials.
A clean, static dissipative fabric might best be made from uniform, moderately conductive fibers, but such fiber materials are rare and overly expensive. Conductive fillers, such as carbon or metal fibers, are less desirable as abrasion tends to release them to become conducting contaminants of particular concern to the microelectronics industry. Conductive coatings are susceptible to abrasion and delamination. Conductive lines or a grid of such lines are sometimes used in fabrics, but charge can accumulate between the lines and then discharge abruptly. Exposed conductors can abrade and produce contaminating conductive particles. Conductors surrounded by insulation do much less to conduct electricity; they serve as the other plate of a capacitor (formed by the charged surface of the insulator and the conductor) which can discharge suddenly under certain conditions.
Resistivity measurements
The draining of charge during wiping comes from both surface and volume conduction. Materials for use in areas sensitive to ESD are often tested for surface or volume resistivity to distinguish static dissipative from conductive or insulative materials. Figure 1 shows the basic elements of surface resistance measurement: a constant direct-current voltage (V) is applied by electrodes to the material under test (e.g., a fabric) and the current (I) along the surface is measured, from which is derived a resistance, R=V/I. From R and the geometry, the surface resistivity can be calculated, assuming all the current travels along the surface. The resistance unit is ohm, or the resistivity is reported as ohm/square, where a geometrical correction factor is used to standardize it to the resistance that would be measured between two parallel conducting lines of length L and spacing L. Surface resistivity will depend strongly on adsorbed molecular layers, principally water, which is why the relative humidity needs to be specified and controlled for such measurements. (Examples exist of a doubling of electrical surface resistivity for every 5% RH decrease in relative humidity.) Some anti-static treatments rely on absorbing water vapor and may be ineffective at humidities much lower than 50% RH. Temperature plays a lesser role, generally.
Surface resistance measurements actually measure a combination of surface and volume resistivities. Some of the current flows through the body of the specimen, some through the opposite surface of the specimen, and some through the base of the sample holder. A fabric made of homogeneous material can have its resistance measured with little ambiguity, although it will depend somewhat on its thickness and on the resistance of the base. If many parallel conductive wires are added to the surface of the fabric, then the measured resistance will depend on the orientation and spacing of these wires and of the test electrodes. How quickly a charge is drained away from the fabric or spread over the fabric would depend on the location of the charge and the locations of the wires.
The resistance of an object often depends more on its volume resistivity than on the surface resistivity. The volume resistivity can be determined from the relationship R = (rho.v) L / A, where R is the resistance (ohm), rho.v is the volume resistivity (ohm-cm), L is the length (cm) and A is the cross-sectional area (cm) of the cylindrical sample being measured. Volume resistivity is a bulk phenomenon, less affected by the test environment than surface resistivity is. Volume resistivities for metallic conductors are typically much less than 104 ohm-cm. Volume resistivities for common insulators are typically much greater than 1011ohm-cm. In between are the static dissipative materials (ESD Association, 1994).
The volume resistivity of polymer fabrics is c. 1014ohm-cm, which is very insulative. The volume resistivity of ultra-pure water is 107ohm-cm. The resistivity of water in equilibrium with typical room air is much less than 1Mohm-cm, due to the carbon dioxide/carbonic acid equilibrium. Anything more than trace amounts of water can be expected to make the fabrics static dissipative, but typical cleanroom humidities are not sufficient. Fabrics like those used in cleanrooms have been found to generate substantial electrostatic discharges (Hirakawa, 1975).
Some fabrics claimed to be “ESD-safe” are made by adding parallel conductive lines or a grid of such conductive lines, often visibly darker than the rest of the fabric. Goodwin (1988) reported measurements made on cleanroom garment fabrics with conductive grids that gave surface resistivities of 105 to 1010 ohm per square and on garments with parallel (“vertical”) lines that gave surface resistivities of 108 to 1013 ohm per square. Thus, most were not static dissipative.
Wilson (1994) reported his measurement of surface resistivity, charge decay and spark discharge for plain weave fabrics made from polyester, polyester surrounding a conductive core material, polyester with a conductive material sandwiched as the width of the fiber diameter, and Nylon with a conductive coating. Discharging occur red from each of these fabrics to at least some of the conducting spheres brought close to the fabrics during the spark discharge test. (No discharge to the conducting spheres occurred when the conductive fibers were groun ded.)
Unlike conventional metallic re sis tors, insulating materials often have resistances that display time dependence, typically being proportional to the time duration raised to an exponent be tween 0 and 1 (ASTM, 1993). We have observed this behavior in surface and volume resistivity measurements of polyester wipers and polyester-based sheet material, Mylar. For some insulators, the resistance is smaller at higher applied voltage gradients, so that R is really a function of the time and voltage gradient.
“Static decay” measurements
Rather than determine a material`s resistivity, some prefer to measure the time history of voltage or electric field after the material has been charged. Various tests have been used to measure such “static decay.” The samples are raised to a high voltage (e.g., ࠹kV DC), perhaps by conduction from contacting electrodes, or by air ionization or by triboelectric charging. The sample is grounded and the “voltage” (often actually the electrostatic field) decay versus time is measured by a non-contacting electrostatic field meter. The time constant is due to the capacitance and resistance of the sample. As noted, the resistance is some mix of surface and volume resistance effects. Homogeneous materials might be successfully tested this way, but non-homogeneous materials, such as fabrics with a mix of conductive and non-conductive fibers, are much harder to measure and interpret correctly, partly due to field suppression by the conductors (Kolyer, 1996; Baumgartner, 1995). A charged fabric adjacent to a conductive surface may produce little electrostatic field on the side away from the conductor, due to compensating charge migration in the conductor. Interpretation of static charge decay results and extrapolation to conditions of interest are difficult, especially in the light of time-dependent and voltage gradient-dependent resistivities.
Swabs
Swabs have small pieces of foam or fabric (“heads”) attached to the ends of handles. The swab`s electrical resistance is some combination of surface and volume resistance for the fabric and surface and volume resistance for the handle to the point of contact, followed by the contact resistance to the glove, thence to the finger, hand, etc. For safe static dissipation, the resistance should be less than that of an insulator and more than that of a conductor. Measuring that resistance correctly involves paying attention to the area of contact in the test. Normal swab use involves a contact of substantial gloved finger surface area (in comparison to the cross-sectional area of the swab), so that the contact resistance in actual use is relatively small. Testing of swabs by attaching metal clips to the handle produces a relatively small contact area that may introduce substantial contact resistance, so large deformable or conforming electrodes are preferred (Cooper and Linke, 1998).
Concluding remarks
The sensitivity of microelectronic components to electrostatic discharge is expected to increase, making the use of static dissipative materials in the manufacturing cleanrooms progressively more important. Measurement of the volume and surface resistivities of cleaning materials will likely become more important as selection of static dissipative materials is emphasized. Such measurements will be needed to supplement the conventional contamination control measurements of particle release, fabric absorbency and the leaching of various species in the presence of various liquids.CR
References
1. G. Baumgartner, “Electrostatic Decay Measurement Theory and Applications, ” ESD Association and IEEE EOS/ESD Symposium Proceedings, Las Vegas, NV, 1995, pp. 262-272.
2. D.W. Cooper and R.C. Linke, “Improved Method of Measurement of Swab Handle Electrical Resistance to Aid in Cleanroom Electrostatic Discharge Control,” 44th Annual Technical Meeting of the Institute of Environmental Sciences and Technology, April 26 -May 1, 1998, Phoenix, AZ.
3. EOS/ESD Association Standard, “Surface Resistance Measurement of Static Dissipative Planar Materials,” EOS/ESD-S11.11-1993. Electrostatic Discharge Association, Rome, NY., 1993.
4. ESD Association, “Advisory for Electrostatic Discharge Terminology – Glossary,” ESD-ADV1.0-1994. Electrostatic Discharge Association, Rome, NY, 1994.
5. B.F. Goodwin, “Clean Room Garments and Fabrics,” in D.L. Tolliver, Ed., Handbook of Contamination Control in Microelectronics, Noyes, Park Ridge, NJ, 1988.
6. T. Hirakawa, “On the Characteristics of Discharges from Electrostatically Charged Cloths,” Polymer J., 7 (3): 269-276, 1975.
7. J.M. Kolyer, “Why Drain Time is Important to ESD,” EE – Evaluation Engineering, August 1996: 72-82.
8. O.J. McAteer, “Electrostatic Discharge Control,” McGraw-Hill, New York, 1990.
9. P. G. Strupp, “An Introduction to Magnetoresistive (MR) Heads: Design, Processing, and Applications,” IDEMA DISKCON `96 USA, San Jose, CA, September 1996.
10. D. Waid, “Manufacturing Yields: Trickle Down at Work in Hard Drives,” Data Storage, October 1996: 28, 30; and personal communication, November 5, 1996.
11. N. Wilson, “The Electrostatic Behaviour of Clothing Fabrics Containing Electrically Conducting Threads,” IEE Colloquium (Digest), Institute of Electrical Engineers, London, n. 041, pp. 6/1-6/5, February 15, 1994.
Dr. Douglas W. Cooper is director of Contamination Control at The Texwipe Company LLC (Upper Saddle River, NJ). He has more than 30 years of experience in the environmental sciences, having been employed in this area also at Harvard University, where he received his Ph.D. in applied physics, GCA/Technology and IBM. He has published more than 100 technical articles, served on the editorial boards of several journals and on several technical society committees, and was elected a Fellow of the Institute of Environmental Sciences and Technology.
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A surface resistivity probe. Photo courtesy of The Texwipe Co.
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Possible current paths during wiping include grounded mat to apparatus to wiper to glove to hand to wriststrap to ground.
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An ESD-safe swab with MR head disk drive. Photo courtesy of The Texwipe Co.