by Lisa Coleman
Finding a cleanroom-compatible electrostatic discharge control solution
The electronics industry is taking a big hit. It is losing billions of dollars annually because electrostatic discharge (ESD) wreaks havoc on delicate circuitry, substrates and disk drive components. As little as 5 volts can damage the smallest and most delicate component and fewer than 100 volts can waste entire lots of electronic circuitry.
With ESD affecting yield, quality and reliability, cleanroom managers are striving to find solutions to ESD events before their production lines are hit with catastrophic failures and yield loss.
Ironically, it is ESD control materials that often create contamination static-dissipative materials are sources for particle or chemical contamination in a cleanroom. If the facility manager's goal is a cleanroom composed entirely of items made from static-dissipative materials to prevent ESD and electrostatic attraction, then that manager needs resourcefulness and a good ESD education. Finding ESD control materials compatible with a cleanroom is a challenge.
However, solutions to ESD problems are more complicated than buying hard-to-find off-the-shelf static dissipative products. In all cases, experts recommend an ESD control program.
There are products that are designed for static dissipation: swabs and wipers, adhesives, coatings, wall materials, wrist straps, gloves, garments and some manufacturers provide clean, static-dissipative work surfaces. However, static-dissipative materials can cause contamination, and many cleanroom products are made of plastics or stainless steel that hold or conduct electricity.
“I'd like to see equipment and products with as much attention to ESD as contamination control,” says Russ Moulton, project leader for Perkin-Elmer's yield improvement group in Santa Clara, CA, which manufactures amorphous silicon. “I'd like cleanroom suits and substrate cassettes made out of static-dissipative materials so everything has a path to ground.”
Other manufacturers, especially in the disk drive industry, echo Moulton's call for more static-dissipative cleanroom materials. With a competitive technology roadmap predicting areal densities of more than 100 percent in the upcoming year, the disk drive industry is desperate for ESD control to keep up with their technology leaps.
“We have to move ahead of what's tried and approved of,” says an engineer at a major disk drive manufacturer.
In the past the disk drive industry was feeding off of ESD technology used by integrated circuit (IC) manufacturers, but now the tables have turned. “For years that was good enough, now it's not,” he adds.
Manufacturers of ESD products are having difficulty finding the raw materials for creating static-dissipative products necessary for protecting extremely sensitive technology such as giant-magnetoresistive heads. IC manufacturers are having a tough time, too. As IC feature size decreases, ESD sensitivity increases. Only in the last three years have electronics manufacturers begun demanding more static-dissipative products.
Responding to reports of ESD damage on reticles two years ago, Asyst in Fremont, CA, developed a new generation of reticle pods, reports Sheng-Bai Zhu, Asyst senior scientist. The standard mechanical interface (SMIF) manufacturer produces reticle pods for particle, chemical and static electricity control. The static-dissipative reticle pods protect the reticles from ESD damage by providing electric paths for charges flowing to ground. Even if grounding is not available, such as during handling, charges are redistributed along the pod surface. As a result the internal field vanishes, eliminating or minimizing the possibility of field-induced charges.
The search for the right raw materials can be a problem, Zhu adds. The reticle pods are made of plastic, which have good abrasion, low outgassing and good flame and chemical resistance and are, therefore, cleanroom compatible. Finding the raw materials for applications that require transparency was difficult, says Zhu. Additives or copolymers created opaque plastic due to different refractive indexes. In addition, the plastics must have the right surface resistivity for these types of applications.
Removing contaminants from the cleanroom environment becomes difficult once a surface is charged with an electric field. Microscopic contaminants can be attracted to a silicon wafer's surface and cause defects and reduce yields.
“One of our biggest problems is finding ESD-control materials that are cleanroom compatible,” says Moulton of Perkin-Elmer. Part of his job entails minimizing ESD risk in equipment manufacturing as well as checking that equipment is grounded. The company produces large glass substrates, which attract charges quickly. Because the substrates are so large, any contamination can cause a drop in yield. “For us it's make or break; the substrate has to be perfect,” he adds.
Like other contamination control managers, Moulton constantly searches for new materials that may provide solutions to ESD control. “I try to find any materials in the fab that we should replace with more static-dissipative materials, such as getting rid of traditional swabs and wipers and using more static dissipative ones,” Moulton explains. “For people like us, there's not a lot of off-the-shelf solutions that we can plug-and-play.”
Moulton credits networking within the Silicon Valley ESD Association for leading him to the cleanroom-compatible materials he uses in his cleanroom.
Why are there not more cleanroom-compatible ESD products available? During the last three to five years, manufacturers supplying products to the clean manufacturing industries have recognized the need to control ESD because their customers are asking for those types of products, says Kay Adams, president of TW Clean in Carlsbad, CA. Finding ESD control products that can be used in any cleanroom is a problem, she says. Many companies are scrambling to get their ESD products certified and tested for cleanroom use.
The emitters for the room ionization systems are visible hanging from the cleanroom's ceiling. All ionization equipment provided by SIMCO. Photo courtesy of Richmond Technology.
One of the biggest ESD control problems in a cleanroom is personnel moving around the cleanroom. “You have a huge mass wrapped in polyester, it creates a huge charge,” Adams says. In a cleanroom, the only way to combat this electrical charge is to ground everything, she adds.
During the last three years Adams and TW Clean co-founder Michele McSwain researched fibers and mill weavers resulting in the production of an electrically groundable cleanroom garment system controlling both ESD and particle contamination for an ISO 5 (Class 100) cleanroom. “You have to protect the product from the human mass and the charge it generates.”
All panels of TW Clean's garment have uniform electrical connectivity and are electrically connected from the top of the hood through the soles of the booties. When hooked up to a ground connection or to ground through a continuous monitor, a cord is attached from the hip. Cleanroom personnel wearing this system can be grounded via the conductive bootie soles when walking on a grounded cleanroom floor without wearing ground cord tethering.
Adams says her garments were constructed in response to the huge demand for products that can help overcome poor yields and high failure rates caused by ESD in cleanrooms. She believes cleanroom product manufacturers will be responding to this need and more off-the-shelf products will be available in the near future. “The whole cleanroom industry will look different in one to two years,” she says.
There are no cheap or quick fixes for ESD control but there are several ways to keep it under control in a cleanroom. Cleanrooms at IBM Corp. in Poughkeepsie, NY, have stainless-steel perforated bench tops, laminar flow and good ionization systems for controlling ESD, says John Kinnear, ESD Association president and an engineer at IBM. Another way to control ESD is to use ionizers installed near material handling locations. As IC features get smaller and ICs become more sensitive to ESD: “You'll have to change how the protection networks work,” says Kinnear. “You're going to make the chips more sensitive to static and you put the burden back on the process people to protect the chips while you are handling them. It's already happened at my site.”
The denial factor
Both manufacturers of contamination control equipment and industry associations find that some electronics manufacturing companies are not up-to-speed on how ESD causes problems and how products touted as ESD solutions can cause further contamination to a cleanroom.
“It's ignorance. People say they don't have a problem,” says Larry Levit, director of technology at Ion Systems in Berkeley, CA.
Recently, Ion Systems was called in to help a semiconductor mask company with a yield problem. Abnormalities were found on the masks but the manufacturer did not recognize the contamination problem caused by ESD until Ion Systems discovered it. “In semiconductor manufacturing, one of the biggest problems is ESD on the reticle. It causes destruction of microstructures so that you're manufacturing non-functioning products,” adds Levit.
In electronics manufacturing, ESD causes different problems such as physical damage where an ESD event may have caused an oxide failure or a metal melt; or more commonly, objects emit high-frequency radiowaves that can cause equipment to act as if it has bad software. Finally, static electricity generates a static field that attracts contamination out of the air and onto a product. Levit summarizes: “it's air ionization or denial.”
The ESD Association in Rome, NY, is filling the educational niche for teaching high tech manufacturers about ESD. “We try to provide the education, means, standards or whatever we can in order to help protect electronic devices,” says ESD president Kinnear.
The association has recently upgraded its Web site with pages and pages of educational materials (www.esda.org) and research resources. However, while the association's charter highlights education, it is also tasked with advancing the theory and practice of electrical overstress avoidance necessary for chip designers, process manufacturing professionals and anyone who handles sensitive devices.
The ESD Association and the American National Standards Institute (ANSI) recently approved the ANSI/ESD S20.20 standard intended to be the commercial replacement for Mil-Std-1686. According to the document its purpose is to “provide administrative and technical requirements, as well as guidance for establishing, implementing and maintaining an ESD control program.”
An operator converts ultraclean packaging on a custom-built machine that uses static control bars and a pulseflow system to reduce charge generation on the material moving through the machine. All static control equipment is provided by SIMCO. Photo courtesy of Richmond Technology.
The standard applies to equipment susceptible to damage by electrostatic discharges greater than or equal to 100 volts human body model (an ESD testing model). In addition, the standard lets manufacturers write their own ESD control program. “It doesn't lock you into doing things one certain way,” says Kinnear. The industry has lauded the standard's flexibility and the ESD Association will be presenting it around the country this year. Last month, the association presented the standard at CleanRooms East 2000 in Baltimore. ANSI and the ESD Association are seeking ISO 9000 audits to certify manufacturing sites to the new standards, says Kinnear. (See “Wanted: ISO 9000 auditors to conduct ESD control program,” CleanRooms, March 2000, p. 4). Facility certification to an ESD standard is not available now but the association hopes to have one trained ISO registrar and launch the first on-site ESD program by September.
As part of its educational initiative, the ESD Association is modifying its handbook to include the 20.20 standard. The handbook should also be available by September, Kinnear says.
Tomorrow's ESD control
With the acceptance of the new ESD standard, electronics manufacturers can more easily develop control programs tailored to their needs. Experts claim ESD control plans should include training, compliance verification and ESD control program plan technical requirements (such as grounding and bonding system requirements, personnel grounding, packaging requirements, etc.). Kinnear adds: “What it really comes down to is a good understanding of ESD, a good practical understanding of your product, a good understanding of where your product is going, and putting it all together with your contamination control people in your back pocket.”
ESD causes and effects
By Ed Weggeland, ESD Association vice president
Electrostatic discharge is the spark felt when you touch a metal object after you have walked across a floor. Below 3000 Vdc, humans will not feel static discharge, however, an electronic part will either be destroyed or have a latent failure occur.
All three photographs show damage caused by an ESD event. Photo A is a scanning electron microscope picture showing the melting of a metal stripe of an MR head. Dimensions of this metal stripe are already under 1.0 micron and will shrink in the next generation. Photo B shows irreversible damage to an electronic circuit. Photo C shows reticle damage. The dimensions of the reticle have shrunk below 1.0 micron. The result is that less and less energy in the ESD event is required to cause damage. Photos courtesy of Ion Systems.
Surfaces separating and friction or movement of materials generate static charges across surfaces. A surface object picks up additional electrons from the surface that it separates from and becomes negatively charged (more electrons than protons). The surface that has given up the electrons becomes positively charged (more protons than electrons). Both charges are easily read by simple electrostatic field meters.
Static charges accumulate on the surface of non-grounded conductors (human skin, the surface of a conductive tote box) and on insulative (non-conductive) surfaces such as practically all common plastics and textiles. These static charges do not move, they “stay put.” With no path to ground, the charges do not move, hence, the charge is labeled “static.” When a non-grounded conductor of static charges (a human) comes close to a ground plane, an ESD event occurs a spark will “jump” or arc from a point on the non-grounded conductor to the grounded object. An ESD event can also occur when a charged insulator comes close to a conductive object, the charge can be induced onto the conductor, which then can rapidly discharge to a ground plane or another conductor.
An ESD event causes a rapid electron movement through microscopic conductive paths within a device and the resulting heat spike from this rapid discharge “melts” or causes damage to “gates” or other insulative parts in the electronic device. If the discharge is large enough, the device will be destroyed and the defect will be found during testing. If, however, the discharge is not large enough to cause destruction, it might just “wound” the device resulting in latent failure. This type of failure will not be identified during device, component or board testing operations. The failure will occur later during use in the customer's application.