Holistic ESD management

With line widths dipping below 90 nanometers, chip speeds moving through the three gigahertz clock speed and barrier ESD thresholds going even lower, fab owners have found a new mantra in S20.20

By Hank Hogan

Like the actual phenomena of electrostatic discharge (ESD), the latest set of control tools is largely invisible.

And while there continues to be new product developments in terms of tangible items—fabrics, wrist straps and static-suppressing air ionizers—most of the latest ESD prevention action is focusing on systems, or practices, set in place to ensure that ESD abatement attempts protect the products that these systems were actually designed to protect.

By far, the biggest evidence of this more systematic and holistic approach to ESD control is the S20.20 standard from the Electrostatic Discharge Association (ESDA; www.esda.org).

The S20.20 provides a formal, consistent process standard that can be ISO 9000-audited, and details education on ESD control, the need for workstation definition, and the requirement for initial worker training and periodic retraining as well as third-party certification.

According to past ESDA president John Kinnear, a senior engineer with IBM Corp. (Armonk, N.Y.), “The 20.20 standard document is downloaded from the ESDA Web site 700 times a month, at a rate that does not appear to be slowing.”

As of this spring, between 35 and 40 sites had achieved third-party certification to S20.20; Kinnear forecasts there will be 100 certified sites worldwide by the end of the year.

According to the ESDA, the push for S20.20 is being driven by a number of factors. Original equipment manufacturers (OEMs) are beginning to demand compliance of their vendors because the standard includes a certification component that reduces or eliminates the need for OEMs to scrutinize and verify a number of different ESD-related standards and specifications. In this case, implementing S20.20 actually cuts cost and effort, making an impact on the OEM's bottom line.

This simplification also pays for those implementing S20.20. David Swenson, president of Affinity Static Control Consulting (Round Rock, Tex.), tells of an Asian contract manufacturer who worked with a dozen companies in the United States and Europe. Each of these customers had a different ESD standard. “Listing the different requirements filled several books, and switching between those specifications took a week. During the changeover, the manufacturing line had to be shut down,” says Swenson, who also conducts S20.20 pre-audits. “That's costing someone money.”

Hind and foresight

Swenson lists training as being the most universally troublesome issue his pre-certification audits have uncovered. This includes initial training, follow-up retraining on a periodic basis and the documentation needed to prove that both took place.

Ron Gibson, a global engineering consultant with Celestica (Toronto, Ont.), adds that when done properly and documented, training is often incomplete because it is not extensive enough.

“A good example that a lot of people overlook is the accounting department,” he says. “Normally, during an annual physical inventory, somebody counts the parts. So, if your accounting people are the ones who have to go and do that, then they must be trained.”

Documentation is another problem area identified in both pre-audits and certification. It includes making sure that the specifications describe what is actually done in practice in a facility, as opposed to what is done in theory. Documentation also includes the proper description of workstations and work places. A facility may have many different work areas, ranging from a fairly dirty final packaging site to a pristine cleanroom. In such a scenario, the two areas will have vastly different work place requirements that must be accounted for in the documentation.

“Each of those types of workstations has to be described in the plan if they have different application requirements and different specifications for the materials,” says Affinity's Swenson.

Control items—the wrist straps, grounding chains and other products that actually dissipate and control static—are another fairly common problem in S20.20 certification and audits. Control items, adds Swenson, have been in use for years without verification that they actually work.

Celestica's Gibson gives the example of a metallic cart used for the transportation of products inside a cleanroom. These carts often have a metal chain dangling from them. In theory, the chain acts as a charge conduit, allowing static to disperse without building up. In practice, however, the situation may be different.

“In a lot of cases, that drag chain doesn't work because it's just too light of a component to make good contact with the floor and get a good electrical connection,” Gibson explains. “But companies often don't know that.”

The difficulty arises because documentation may state that the carts are grounded to the floor through a drag chain. When actual measurements are done, however, there's a sudden realization that the control item isn't working. Gibson recommends that selected technical elements be verified to ensure that they function in practice, and that there be ongoing tests to make sure that these measures continue to work.

This simple verification avoids a major non-compliance to the standard. The same can be said for having the proper training and documentation in place.

Less than one

In addition to S20.20, there are other signs that ESD control is moving toward an all-encompassing approach. Over the past few years, new monitoring tools have been deployed, and there's been an increasing integration of ESD abatement and measurement with other process control and monitoring.

Simco (Hatfield, Pa.), for example, is focused on fully integrating ionizers within tools and the manufacturing environment. According to business unit manager James Curtis, the company's latest entry—ScorpION—combines digital ionization technology with RS-485 communication and control capability. It also uses infrared for remote control setup.

In the realm of monitoring and testing, transmission line pulse testing (TLP) is increasingly being used to check for ESD susceptibility of semiconductor devices. In TLP, a tester applies a pulse and then repeatedly increases it in magnitude until the device under test fails.

Steven Voldman, an IBM engineer and scientist, notes that this approach offers current (I) and voltage (V) information that other ESD failure-testing methods can't provide. “It's really a pulsed IV characteristic, where you can determine the device characteristics or product characteristics in terms of snap-back voltage, reverse-current phenomena and the current and voltage to failure,” he says.

This information is useful in designing devices with improved ESD resistance and in characterizing field device failures that occur. Such data will become increasingly important as semiconductor device features shrink. As semiconductor technologies go below 90 nanometer feature sizes and chip speeds move through the three gigahertz clock speed barrier, device ESD thresholds are going lower and lower.

A glimpse of what may lie ahead can be seen in the disk drive industry. To squeeze more data onto a disk, manufacturers have had to deploy smaller and more ESD-sensitive drive heads. The result has been devices that blow up when workstation surfaces climb above one volt. That's at least an order of magnitude and possibly several more orders of magnitude sensitive than current semiconductor devices.

According to Larry Levit, chief scientist at Ion Systems Inc. (Berkeley, Calif.), an air ionizer works at about 20,000 volts and produces positive and negative ions. To get the work surface to stay under one volt due to a build-up of static charge, the air ionizer control has to be much less than one volt.

That's tough to do, but the alternatives—such as the use of non-voltage-dependent methods involving radioactive sources—aren't attractive to customers. It's an ESD-control situation that semiconductor devices will approach as feature sizes shrink. “Pretty soon, conventional technologies aren't going to cut it,” says Levit.

These technology trends are linking up with basic market factors, forcing ESD abatement techniques that are more systematic in nature. Celestica's Gibson notes that a decade ago, simply having workers wearing wrist straps and the presence of packaging to control static was enough to satisfy many customers. But that's changed as expectations of ESD control has matured.

“Now it's to the point where they [customers] are really looking to ask, 'Is your process capable of handling this sensitive part?' And in turn, 'How are you going to prove it?'” —.Ron Gibson, global engineering consultant, Celestica
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As Gibson concludes, “Now it's to the point where they are really looking to ask, 'Is your process capable of handling this sensitive part?' And in turn, 'How are you going to prove it?'”

Hank Hogan, a special correspondent to CleanRooms magazine, is based in Austin, Texas. He can be reached at: [email protected]

Static electricity: Creating charge

Static electricity is defined as an electrical charge caused by an imbalance of electrons on the surface of a material. This imbalance of electrons produces an electric field that can be measured and that can influence other objects at a distance.

Electrostatic discharge is defined as the transfer of charge between bodies at different electrical potentials. Electrostatic discharge can change the electrical characteristics of a semiconductor device, degrading or destroying it. Electrostatic discharge also may upset the normal operation of an electronic system, causing equipment malfunction or failure.

Another problem caused by static electricity is that of electrostatic attraction. Charged surfaces can attract and hold airborne contaminants, making removal from the environment difficult. When attracted to the surface of a silicon wafer, a device's electrical circuitry, photolithography masks, and reticles, these particles can cause random wafer defects and reduce product yields.


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