Electrostatic discharge (ESD) negatively affects production, yields, manufacturing costs, product quality, reliability, and profitability. Static-sensitive magnetoresistive (MR) drives and giant magnetoresistive (GMR) heads can be destroyed by ESD (Photo 1A). In the semiconductor industry, especially as IC geometries shrink and circuits become increasingly complex, ESD can cause sparking and damage (Photo 1B). The hidden costs of shutdowns, malfunctions, material waste, and equipment repair make an investment in ESD control a cost-effective means of achieving higher quality and improved yields.

Many effective means of controlling ESD have been developed. To date, however, no single method offers complete protection. Certainly ionization helps, as does the common practice of using wrist straps and footwear to ground the operator's body. But even using both approaches together is not enough to ensure maximum protection; instead, the body and the whole garment system must be grounded.

Every move an operator makes causes contact, separation, and rubbing (friction) between cleanroom garments and personal clothing, generating a triboelectric, or frictional, charge in which electrons are transferred from one material to the other. Simple body movements can cause a garment to generate 3500 V or more of static electricity; compare this to the 2 V limit specified in the disk drive industry.

The meticulously controlled cleanroom environment, with its reduced levels of charged particles and humidity, actually promotes the production of high static charge levels. People in the cleanroom, and the work they do, produce particulate contamination and static electricity. When particles become positively or negatively charged, they seek to cling to surfaces that are charged opposite to their own; a positively charged particle will seek a surface that is negatively charged, and vice-versa. Without extra particles to neutralize them, particles will attach themselves to products, equipment, furniture, and tools, as well as to people and their garments.

An example of this phenomenon is reflected in Photo 2, which shows the results of a contamination study in a Class 1 minienvironment. While the product (a silicon wafer) would not normally be exposed in any cleanroom environment for six weeks, the study does clearly show that many more particulates are attracted to an ungrounded wafer than to a grounded one, in which the attraction of particles is neutralized. This applies to any surface, including a garment. A grounded garment, therefore, will remain cleaner than an ungrounded one.

Because charges in a cleanroom take a long time to dissipate on their own, some intervention must take place. Figure 1 shows that, without intervention, a residual voltage of 5000 V would take almost 34 hours to dissipate.

Although we can ground people effectively, grounding people does little to eliminate the ESD generated by their garments. Providing a path to ground for the garment, however, can safely eliminate these charges. Garments for ESD control and garments certified for cleanroom use (particulate control) have long been available. To date, however, the challenge has been to develop a single garment that is both cleanroom compatible and ESD capable.

For ESD control, it is essential that all panels or parts of any garment or garment system have electrical continuity to ensure groundability with a single grounding connection or grounding point. If, for example, the sleeves of a garment were not electrically connected through the seams joining them to the body of the garment, each sleeve and panel of the garment would have to be grounded separately.

Traditionally, knitted fabrics have been used for making non-cleanroom certified ESD garments. Their relatively loose weave provides for easy airflow, making the garments more comfortable to wear. Conductive fibers incorporated into these knit fabrics are readily exposed, allowing for easy electrical connection through sewn seams.

Unfortunately, however, the "open holes" inherent in knit fabrics do not provide the particulate barrier offered by woven cleanroom garment fabric. Photo 3 shows a comparison between the construction of a knitted ESD control fabric (2A and 2B) and a tightly woven cleanroom ESD control fabric (2C and 2D). The photos illustrate how the conductive fibers are buried in the woven cleanroom fabric.

To ensure electrical continuity, the conductive fibers must be reconnected at the sewn seams. This occurs easily enough with knit fabrics because there are so many opportunities for connection with the conductive fibers so readily exposed. The "buried" conductive fibers in the woven cleanroom fabrics, however, are very difficult to reconnect, and it is difficult to maintain electrical contact through the life of the garment, which includes wearing, stressing of seams, stretching, and numerous washings. If this continuous electrical connection could not be assured through the life of the garment, the garment or garment system could not be safely grounded.

Solving the Problem
As owner of an ESD control garment company, co-author Kay Adams (Photo 4), had served on the ESD Association standards committee for many years and recognized the need for an effective ESD garment for the cleanroom environment. Michele McSwain (Photo 5), president of a disk drive recording head manufacturing company, had experienced the realities of non-performing cleanroom garments first-hand and recognized the need for im-provement in ESD and particle control. Working closely with fiber manufacturers and a fabric weaver, they developed the first true ESD certified, Class 100 cleanroom compatible, monitorable, groundable garment.

The new garments provide continuity of the grounding path from the top of the hood to the soles of the foot-wear. A proprietary seaming technology places conductors in the seams, assuring panel-to-panel electrical continuity through the entire garment system. Built-in redundancy provides continuous performance and reliability. A lightweight grounding connection at the waist (Photo 6) grounds both the operator and the garment system, replacing the need for tethering cleanroom personnel to ground by a wrist strap that reduces mobility and functionality.

Patented dual isolated grounding paths are designed for use with dual continuous monitoring systems, allowing production to stop immediately, should the garment or the wearer disconnect from ground. Probes can be attached to meters to monitor, measure, and record static events, enabling the tracking of yield problems. This is especially beneficial in the disk drive industry, where only minute amounts of static can cause major negative results.

If complete suitsÑincluding hoods and bootiesÑare worn, no ground cord is needed, since all parts of the garment system and person are connected to, and grounded through, the conductive soles of the booties to grounded flooring (Photo 7).

Conclusion
Historically, the testing for cleanroom garment attributes has been limited to particulate control issues. There have been no recognized test methods or performance standards for measuring the ESD characteristics of cleanroom garments. The ESD Association's STM2.1-1997 for Static Control Garments has not been applied to cleanroom garments, largely because of the inability of present cleanroom garments to meet this performance specification, which requires a garment to have panel-to-panel electrical continuity. In the past, only fabric samples were measured, with no attention paid to the ability of a whole garment to dissipate a charge to ground. Now that cleanroom garments can be manufactured in an ISO 9001 facility for Class 100 compatible particle control and ESD protection, we can assume new standards will be developed. Given the correlation between improved particle control and grounded surfaces, product yield and profitability, the future of a total ESD cleanroom program is likely to include grounded garment systems.

Contact Kay Adams, Co-Founder and President, and Michele McSwain, Co-Founder and Vice-President, at TW Clean, 2205 Faraday Ave., Ste. B, Carlsbad, CA 92009; 760-438-7788, fax 760-438-6868; kadams@techwear.com; michele@techwear.com

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