The forgotten contaminant: Body-borne static
02/01/2000
A "total system approach" to clean-room garments protects against contaminants and electrostatic discharge.
CHRIS ADAMS
KAY ADAMS
While most clean-room garments are effective containers for body-borne contaminants, they often generate damaging electrostatic charges themselves. The ideal solution: a "total system approach"1 - a suit grounded from hood to booties.
Why is static discharge dangerous? Circuits, disk drives and other microelectronics have become more intricate and are, therefore, sensitive to seemingly minute amounts of electricity. Consider that the smallest discharge perceptible via human sensation is about 3000 V. In contrast, many disk-drive producers specify tolerances within 2 V for handling their products. And as if this were not enough for clean-room operators to worry about, a sensitive product does not need to be touched to damage it: If a sleeve, or any other article bearing a static charge, is simply brought near a sensitive product, it can cause serious damage through induction charging.
The ESD Mechanism
If left to nature, a charged surface will typically neutralize as its electron imbalance equalizes. The electron surplus, or deficit, gradually "decays" in a phenomenon called corona discharge. Were it visible to the naked eye, corona discharge would resemble an electrostatic halo or field surrounding the charged surface. Anything brought within this field will either gain or lose electrons, and it is, therefore, subject to the damaging charge via induction. Generally, the greater the charge, the broader and more potentially damaging the field.
While detectable damage because of electrostatic discharge (ESD) events initially seems to be the most catastrophic (Figure 1), it is arguably less costly than the latent variety. If damage is detectable, the loss is limited to materials and labor up until a quality control (QC) check. If damage is slight enough for a product to slip past QC, then the losses may extend to customer service: product repairs, exchanges and, perhaps most significantly, lost confidence in the end product.
Static is not only dangerous for its discharge and induction potentials, but also for its effect on airborne particles in a clean room. Any free-floating particles in its atmosphere will naturally be attracted to a charged surface, whether a computer monitor, a polyester clean-room garment or a silicon wafer (Figure 2). An ungrounded garment will generate a charge and will, therefore, attract and loosely retain such particles.
The Origins of Static
There are typically two sources of static charge: induction and tribocharging. Induction can transfer a charge between two surfaces even though they lack physical contact. Not only can a product by a charged garment or instrument be damaged via induction, but a surface, like a sleeve, can be charged simply by passing through an electromagnetic field. Because major sources of those fields, such as old computer monitors and/or high-voltage sources, are easily identified and removed from sensitive areas, the other source of static charge remains the more difficult to detect and control.
Tribocharging is simply the exchange of electrons between two surfaces that come into intimate contact. If there is high pressure exerted between surfaces - such as through an adhesive like packing tape or Velcro - or if two surfaces are simply rubbed together, the result is an exchange of electrons that will leave one or all surfaces with a charge imbalance.
A household example of an ESD event from tribocharging is receiving a shock from a doorknob after walking across a shag carpet in a dry room; an electron exchange is simply transacted with the carpet and the knob, resulting in an instant equalization of the charge. However, if both surfaces are conductive and grounded, no charge will be generated via contact. On the other hand, if either or both surfaces are insulative, the charge imbalance, or static, will be retained.
On insulated surfaces, such as polyester and Plexiglas, the charge tends to collect into little "islands" of charge imbalance called hot spots. These hazardous areas will remain until they are either slowly radiated away via corona discharge or conducted away by contact with another surface. Therefore, a static charge translates into both an increased likelihood of a damaging ESD event and a surface`s electromagnetic attraction of any particles in a clean-room atmosphere.
Static-free vs. Clean?
Unfortunately, many conditions for ideal particle control run counter to a static-control situation. For example, a moisture-reduced atmosphere in a clean room tends to enhance the possibility for tribocharging, while a tightly woven polyester worn over an operator`s personal clothing reads like a recipe for Figure 2.
Even the stripping of particles from the environment contributes to unwanted charges. While considered to be contaminants in a clean room, naturally occurring particles aid in static decay by being attracted to charged surfaces. While fewer free-floating particles mean fewer electron transactions, the end result is a slower static-decay rate.
Pioneering Static-control Methods
Several measures have been adopted to reconcile the controls of static and particle contamination but most tend to address them separately. An early example was to ground an operator`s body with a standard wrist strap and ground cord leading from the garment`s sleeve. The problem with this solution is that the operator`s body becomes the "grounded eye" of an electrostatic storm. The garment, or bunnysuit, still tribocharges against the operator`s personal clothing and develops the charged hot spots on the surface even though the body is safely grounded. Other problems include the impediment to an operator`s mobility posed by the leashed wrist, as well as the need to tape the ground cord`s exit at every suit-up.
A better solution lies in reducing the fabric`s surface resistivity. The now-familiar grid of conductive carbon fiber in the fabric of most clean-room garments bridges the insulative gaps between any islands. The conductive surface not only reduces the triboelectric effect, but it also increases the surface area over which static can dissipate via corona discharge; this minimizes the charge but does not eliminate it. Even with the conductive grid of carbon fiber woven into the fabric, the charge remains; it is simply spread over a larger surface area. If the garment is not constructed in a way that ensures conductivity across the seams, the surface area is not as large as is supposed. In this case, the fabric panels simply become enlarged hotspots, increasing the charge and, subsequently, the potential damage resulting from discharge, induction or particle attraction.
Nevertheless, ensuring surface conductivity is an important rung in the ladder toward completely eliminating static. Once a garment is conductive from sleeve-to-sleeve and hood-to-booties, safety is simply a matter of securing a ground strap at the bunnysuit`s hip, or for greater operator mobility, using conductive flooring and booty soles. For those with especially sensitive products or processes, constant ground monitoring is available in many cases to assist in isolating possible product faults.
Once grounding is ensured, terms like induction, tribocharging, static decay rates and corona discharge may be expected to fall quickly from clean-room conversations. AP
Reference
1. Institute of Environmental Sciences and Technology, Contamination Control Division, Recommended Practice CC022.1, Electrostatic Charge in Cleanrooms and Other Controlled Environments, 1992, p. 12.
CHRIS ADAMS, technical services advisor, and KAY ADAMS, president, can be contacted at TW Clean, 2205 Faraday Ave., Suite B, Carlsbad, CA 92008; 760-438-7788; Fax: 760-438-6868; E-mail: [email protected] and [email protected].
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Figure 1. A PCB through-hole`s damage via ESD is visible. Other forms of injury via induction may be latent and eventually cause product breakdown in the field.
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Figure 2. Two silicon wafers left exposed in the same class 1 mini-environment for six weeks. Sample A was grounded; sample B was exposed to a moderate charge.