by Andrew J. Magenheim, Ph.D and Carl Newberg

ESD resistance

Some level of charge is always generated during any contact between two objects (e.g., Fowler, 1988). The primary mechanism for reducing ESD damage is dissipation of the charge to ground. Assessment of a material or a product's ability to dissipate charge can be addressed through two types of measurements: charge decay time, and electrical resistance. If the resistance is known, the decay time along the same path can be predicted. This technical feature addresses resistance only — the measurements that are applied to cleanroom gloves and their relevance are reviewed.

The most important considerations in evaluating the resistance of a material are:
• Understanding the electrical path for a given resistance measurement
• Defining the desired best path based on the function of the product or material

Analogous to water flowing in a river, charge dissipation (the flow of electrical current to ground) will occur along the path of least resistance. Even though some products may demonstrate lower resistance along a given electrical path, this perceived advantage has no real benefit if alternate electrical paths have demonstrably lower resistance. As a prime example of this—many industry customers mistakenly base glove selection on a comparison of surface resistance when electrical resistance through the gloves (volume or bulk resistance) is up to two orders of magnitude less than along the surface.

Figure 1. Measurement of in-use resistance. The electrical resistance is measured through the glove and operators’ body.Click here to enlarge image

Understanding the best path for charge dissipation is also critical to selecting the best product for the desired application. Dissipation along the surface may be appropriate for a static-shielding bag, as this effectively prohibits ESD damage of its contents during transport. However, this mode of discharge will not protect static sensitive items in proximity to an operator's gloved hand. In fact, charge dissipation through the glove is preferential as this removes charge from the surface nearest the component or process.

Below, we address the methodology for evaluating the dissipative properties of gloves in the context of what is measured using various techniques. The observations and implications of the results are interpreted in the context of establishing the dissipative performance of common cleanroom glove materials (latex, nitrile and vinyl).

Methods
Electrical resistance is a measure of the ability for charge to flow along a defined electrical path. Formally, resistance (R) is defined as the ratio of DC voltage to the current flowing between two electrodes and is expressed in Ohms (Omega). Resistance results are often sub-divided into insulative (R>10¹¹ Omega), dissipative (10⁴ Omega < R <10¹¹ Omega), and conductive (< 10⁴ Omega) classifications. Insulative materials are inappropriate for use in static-sensitive applications as they retain charge on their surfaces. Conductive materials dissipate charges rapidly and are equally unacceptable. Materials in the dissipative range are desired as the charge is removed in a controlled manner.

Click here to enlarge image

Surface Resistance per ANSI ESD-S11.11. This test method was introduced for packaging materials and attempts to measure the electrical resistance between two electrodes applied to a single surface of the glove using an industry-standard concentric ring probe. It is important to note that this method is designed for measurement of resistance between two electrodes placed on the same surface of a flat material and does not necessarily measure the resistance along the surface. Current flow can occur along the surface through the material or a combination of paths (Baumgatner, 1999).

Volume Resistance per ESD STM 11.12. This test method measures the resistance through the material. The measurement is made between the center electrode of the concentric ring probe and a conductive plate placed on the opposite side of the test specimen.

In-Use Resistance (ESD TR-03). This test method simulates the functional performance of gloves while being used by the operator. It is based on methods developed by several test labs and end-users. The resistance is measured through the glove to the operator employing a standard 5-lb NFPA probe as one electrode and a wrist strap as the other electrode (Figure 1). This test method yields an indication of the dissipative function of the gloves while used, as operators are similarly grounded through use of a wrist strap in all ESD-sensitive operations. The ESD Association is currently in active development of a formal standard for this measurement.

Results
Several factors can influence the measured electrical resistance of gloves and glove materials. The following discussion reviews the following:

1. The electrical path measured by the experiment
2. The material (vinyl, nitrile, latex etc.)
3. The humidity
4. Surface contamination

Comparison of the resistance measurements
Results from the three resistance measurements are directly compared for each of the common glove materials in Figure 2. Several observations are apparent.

1. Natural latex gloves are insulative (> 10¹¹ Omega) for all tests.
2. For nitrile gloves, surface resistance is highest; volume and in-use resistance is well within the dissipative range (10⁵ to 10¹¹ Omega).
3. Vinyl gloves show a similar pattern to nitrile with better performance in all categories.

Figure 2. Resistance of common glove materials measured using surface, volume and in-use methods (100V, 40% RH). All measurements are the mean of six pieces for each product. Source: River's Edge Technical Service Report 99-38, 5/10/99.Click here to enlarge image

Because natural rubber latex gloves are insulative (R > 10¹¹ Omega) they are not appropriate for most ESD-sensitive applications. Both nitrile and vinyl fall within the dissipative range and are acceptable in terms of their dissipative characteristics. While vinyl gloves perform better than nitrile in electrical resistance, plasticizers required in the manufacturing of vinyl gloves represent a defined contamination hazard for many microelectronics customers (IDEMA STD BINDER). For this reason, nitrile is considered the material of choice in cleanroom ESD applications.

For all glove types, surface resistance is greater than the resistance measured through the glove (volume or in-use). This indicates that the preferred path for charge dissipation is through the glove, not along its surface. Thus, surface resistance measurements are virtually meaningless in the context of assessing the dissipative performance of gloves in the cleanroom. Nonetheless, many users rely on surface resistance as the primary ESD selection criterion. The in-use resistance measurement is the best functional measure of dissipative performance of gloves when worn.

Why do end-users measure surface resistance?
Surface resistance is commonly measured for many materials in the cleanroom and electronics industry, as dissipation of charge along the surface is desired for many applications. The standards EIA-541 and ASTM D 257 are written specifically for packaging materials where the concern is protection of the ESD-sensitive parts from externally generated charges. For this function, it is desirable to conduct the charge away from the part along the exterior surface of a bag (Figure 3A). The same argument applies to carrier trays, wafer boats and some other materials.

Figure 3. Schematic representation of the desired path to ground for different applications. Note: In some cases it is desirable to conduct current along the surface, for other applications (including gloves) carrying the charge away from the surface is required.Click here to enlarge image

Is dissipation along the surface desirable for gloves? The exterior surface of the gloves is in closest proximity to the ESD-sensitive parts. Thus, it is not desirable to constrain the charge to this surface. A far more desirable path is through the glove to the grounded operator as represented in Figure 3B. This is the electrical path best measured using volume resistance and in-use resistance.

For nitrile and vinyl, the results (Fig. 2) show that both volume and in-use resistance are significantly less than surface resistance. This indicates that, as desired, charge will be dissipated through the material rather than along the surface.

Several nitrile gloves from different manufacturers were evaluated to assess their performance along different electrical paths (Figure 4). The data indicates that these products are statistically equivalent except for product D. In fact, the difference in in-use resistance between products is barely resolvable between four of the five products tested. Note, in-use performance tracks volume, not surface resistance. This is further evidence that the surface resistance measurement does not reflect the actual dissipative path for gloves.

If a user were to base his choice solely on the surface resistance results, the selection would yield the glove with the worst volume and in-use performance (Fig. 4). Because the current flows along the path of least resistance, this selection yields the worst result at functionally protecting the ESD-sensitive parts. Selection and evaluation must be based on the lowest resistance path to ground and the desired mechanism to protect the operations.

Effect of humidity
As water is a polar molecule and dissipative, humidity is expected (known) to influence electrical resistance. For this reason, the early packaging standards called for resistance measurements to be conducted under controlled humidity at the lowest value able to be maintained (12 percent RH). This served to eliminate the effect of adsorbed water as a variable for comparison of materials.

Importantly, for packaging materials this functionally simulates the dry conditions encountered during airfreight shipment.

As cleanroom gloves are used in controlled environments (40-50 percent RH), evaluation under these conditions is not functionally relevant.

Figure 4: Comparison of surface, volume, and in-use resistance for 5 nitrile gloves from different manufacturers. All measurements are the mean of six pieces for each product. Source: River’s Edge Technical Service Report 99-77, 10/8/99.Click here to enlarge image

Humidity has a profound influence on the measured volume resistance of both nitrile and vinyl gloves (Fig. 5). Natural latex does not appear to be as significantly effected by humidity. Both vinyl and nitrile materials pick up water during exposure to more humid conditions enhancing their dissipative properties. Experiments have shown that nitrile gloves may pick up as much as 1.2 percent water between 12 percent and 75 percent RH. During the development of standards through SEMATECH the glove manufacturers proposed that material property measurements should be conducted at working conditions (40-50 percent RH) to simulate actual conditions.

Hydration of gloves is enhanced through contact with the operator's skin when worn. The moisture created by the sweat of the operator's hand contributes to improving the dissipative properties of the glove. The sweat layer at the hand surface enhances the electrical contact between the operator and the glove further enhancing the electrical path to ground. The effect of hydration of a nitrile glove while worn is demonstrated in Figure 5 where the in-use resistance was recorded over time. Hydration is relatively rapid with equilibration occurring within 15 to 30 minutes. Humidity and hydration effects can explain why the in-use resistance is lower than the measured volume resistance (Fig. 2).

Effect of contamination
Welker, et al. (2000) concluded that heavy particulate contamination can negatively impact in-use decay times due to the contamination creating a higher resistance to the flow of charge. In contrast to this result, a common technique for enhancing surface resistance of materials is the application of conductive “anti-static” agents to the material surface. This approach is the one taken in common applications like reduction of electrostatic cling in household laundry.

Figure 5. In-use resistance versus time on hand. Source: Safeskin TR #20041.Click here to enlarge image

These applications involve the addition of amines, surfactants or other “anti-static” additives to the surface. For gloves, surfactants used in the manufacturing process are excellent anti-static agents and if left on (or added to) the glove surface, will act to lower surface resistance. Manufacturers have used the surfactant approach (either by accident or design) to support claims for enhanced surface resistance of their gloves. This can be seen in the following analysis of an “ESD nitrile glove” before and after washing the glove in deionized water. Upon washing the glove by rinsing in deionized water, surfactant foam is clearly visible indicating the presence of a highly mobile contaminant. After washing, the surface resistance increases 700 fold. Similar experiments with regular production cleanroom gloves from the same manufacturer indicated no change before and after washing.

This will yield a perceived benefit for the customer who evaluates glove properties solely on surface resistance. However, there is a clear trade off of this perceived enhancement with contamination potential that can result in latent yield loss. The trade off between contamination and surface resistance is not a choice any user needs to make. There is no reason to accept a dirty glove to gain lower surface resistance, as the surface path is not the preferred path to ground and will offer no functional advantage for the user.

Conclusion
When considering the functional performance of nitrile gloves, the in-use resistance indicates that there are only limited differences in performance (Figure 3). Despite this observation there are important considerations in selection of a cleanroom glove for ESD purposes.

The fit of the glove is essential as electrical contact can be compromised if the glove does not make consistent contact with the hand. Thus, baggy gloves or gloves that fit poorly in the fingers should be avoided. Poor quality in glove manufacturing can result in the presence of microscopic defect or pinholes. These holes can enhance electrical continuity between the wearer and the glove surface; however, the lack of barrier integrity results in a loss of functional isolation of the wearer from the environment.

• The most important consideration when evaluating the dissipative property of any material or product is understanding the measurements made and the desired path for dissipation. For cleanroom gloves, the following can be demonstrated:
• The desired dissipation path to ground is through the material, as this removes the charge from the proximity of the ESD-sensitive parts to ground (through the operator).
• Comparison of surface, volume and in-use measurements for gloves indicates that the preferential path to ground is through the glove as desired.
• Perceived advantages in surface resistance offer no real ESD advantage—and can result in a real threat due to residual contaminants.
• Poor glove quality as indicated by poor fit and/or pinholes will influence in-use performance.

Overall, in-use performance of nitrile gloves is near the desired level of 1 megohm used to fix the resistance of cleanroom operators.

References

• ANSI/EOS/ESD S 11.11 EOS/ESD Association Standard for the Protection of Electrostatic Discharge Sensitive Items-Surface Resistance Measurement of Static Dissipative Planer Materials. 1993
• ESD DS 11.12. ESD Association Standard for the protection of Electrostatic Discharge Susceptible Items-Volume Resistance Measurement of Static Dissipative Planer Materials. 1997.
• ESD Association Technical Report: Glove and Finger Cots (ESDTR-03-00).
• ESD Journal, Approved test procedure: gloves (ATP #G1000). August 1998.
• ANSI/EOS/ESD S 11.11 EOS/ESD Association Standard for the Protection of Electrostatic Discharge Sensitive Items-Surface Resistance Measurement of Static Dissipative Planer Materials. 1993
• ESD DS 11.12. ESD Association Standard for the protection of Electrostatic Discharge Susceptible Items-Volume Resistance Measurement of Static Dissipative Planer Materials. 1997.
• Baumgartner, B. 1999. Ohms per square answers. Threshold, December 1999.
• Baumgartner, G. 1995. Analysis of ESD Glove Use. EOS/ESD Symposium 97-73. Pg1B.5.1-1B.5.8.
• Fowler 1988
• Welker et al. 2000

Andrew J. Magenheim, Ph.D. is senior scientist at Safeskin Corporation. Carl Newberg is the president and owner of River's Edge Technical Service, an independent testing laboratory and consulting service to the ESD and contamination control industries.