Controlling ESD at the Cleanroom Workstation
ESD control includes training cleanroom personnel, identifying sources of static electricity, preventing charge generation, dissipating charges, and removing insulative materials from the controlled environment.
By Rob Linke
Electrostatic discharge (ESD) can be thought of as yet another form of contamination in the cleanroom environment. Like particles, fibers, ions, residue film and biologicals, ESD can cause yield
loss and field failure in electronic devices. But, unlike other contaminants, ESD is invisible and cannot even be felt at levels low enough to be destructive. In fact, most people only begin to feel ESD at around 4,000 volts, far above the 50-volt damage threshold of sensitive electronic circuits.
Low levels of ESD are particularly difficult to control. Their sources are not only cleanroom personnel, but items such as equipment, garments, wipers, swabs, gloves, furniture, work surfaces and documentation materials. In a well-planned workstation environment, all items in the immediate vicinity of the workstation must act as an integrated system to prevent ESD damage.
ESD control
There are several keys to ESD control in electronic manufacturing environments:
Awareness training for all cleanroom personnel. Training cannot be overemphasized. ESD is not always an obvious problem. Damage can occur without direct contact and at extremely low charge levels. Operators need to understand the damaging effects of ESD, the basic principles of charge generation, and the identification and use of low-ESD materials and devices.
Identification of sources of static electricity. Identifying sources of charge build-up in the vicinity of the workstation can be done through a combination of electrostatic field testing and a knowledge of charge generation. Hand-held field meters are useful in identifying sources of static electricity.
Prevention of charge generation. The prevention of charge build-up relies as much on the overall integrity of the workstation as the materials used in its environs. Properly designed workstations provide a continuous path to safely dissipate generated charges.
Dissipation of any generated charges. By definition, dissipative materials are neither conductors, readily allowing current flow, nor insulators, causing charge build-up. Their conductivity lies in-between these two zones. Dissipators are capable of slowly conducting electrical charges.
Removal of insulative materials from controlled area. In judging materials, one needs to examine the inherent resistivity of a substance. The measurement of surface conductivity is a relatively simple matter involving the use of a surface resistivity meter on a planar surface. Samples need to be “conditioned” at a fixed humidity level over a given time period to allow for consistent measurements. The photo above shows a surface resistivity probe in use.
A cleanroom workstation system might include the following:
1. Dissipative garments for personnel including dissipative gloves and conductive footwear or grounding straps
2. Dissipative work surfaces
3. Grounded seating
4. Ionized air and humidity control
5. Dissipative consumables, tools and other workstation accessories.
By properly designing the workstation layout and choosing proper materials any charges which are generated can be safely dissipated to the ground.
Plastics: A difficult challenge
Perhaps the most difficult challenge for contamination control managers is the removal of insulating materials from the workstation. This category includes virtually all plastics and glass; materials widely used in cleanrooms because of their inherent cleanliness. Plastics are also the foundation of most cleanroom consumables, including garments, gloves, wipers, swabs, dispensers, document holders and a host of other disposables.
Traditional approaches to making plastics electrically dissipative and thus ESD-safe are generally in direct conflict with contamination control requirements. Accomplishing both is difficult.
Carbon loading
An ESD-safe plastic must, by definition, dissipate electrical charges. One traditional approach involves mixing conductive carbon fibers or “carbon black” particles into the plastic resin. Although actual loadings may vary, 20-30 percent is common. The carbon filler is dispersed as a mixture in a random fashion and the matrix of carbon particles or fibers forms random conductive paths within the plastic that are capable of dissipating charges.
Although carbon-loaded plastic materials are inexpensive (carbon is often less expensive then the base plastic resin), ESD-safe at low humidity levels, and readily manufactured, they can cause high levels of contamination. When a carbon-filled plastic is molded or formed, some of the carbon fibers or particles are exposed. This leaves them vulnerable to sloughing (particle generation under abrasion), and, once generated, are migratory and conductive. There is no easy solution to this problem as some exposure of the carbon is necessary to provide a conductive path.
Carbon-filament-core textiles
Another common approach to ESD control is the use of carbon-filament-core polyester fibers in garments and wipers. These materials use a grid of carbon filament fiber in conjunction with a polyester knit. However, since they are surrounded by an imperfect non-contiguous polyester sheath, these fibers also generate carbon particles under stress. Carbon particles released in a minimal stress wet test done on a carbon-filament-polyester can readily be identified as dark particles released against the contrasting filter media. Again, even though an ESD risk has been reduced, the particle contamination burden was increased and the use of these materials could cause serious problems in a controlled cleanroom environment. Additionally, breaks in the grid or poor construction can reduce effectiveness.
Metal loading
Like the carbon loading of plastics, metals added to plastic resins can also contribute to contamination in cleanroom environments. Metal particles are conductive, often abrasive, and when metal is added to plastic resins, some metal particles inevitably protrude through the surface and may slough off. Even without actually separating from the surface, the embedded metal particles can abrade and/or oxidize. An additional problem is that in a metal filled plastic, flexing or an uneven distribution of particles can cause erratic electrical performance.
Topical and blooming antistatic agents
Chemical treatment is another method used to render plastic materials dissipative. Two types exist: topical and blooming. Topical antistats are liquids which are sprayed or wiped onto a surface and act to attract moisture from the environment. A blooming antistat is typically incorporated into the resin mixture and acts by diffusing onto the surface of the plastic. Since both blooming and topical antistatic treatments rely on high ion concentrations to “getter” water molecules from the atmosphere, they thus create a static-dissipative surface. However, these mobile ions can cause serious contamination problems when they transfer off of the surface.
New approaches for plastics: Plastic alloys
New plastic alloys have been introduced that solve many of the problems associated with the traditional approaches discussed above. These inherently dissipative plastic alloys do not rely on carbon, metal, topical or blooming antistats. Instead, conductive polymers are compounded with other plastic materials to create clean and ESD-safe plastic materials. Problems associated with filled or chemical-loaded materials such as sloughing, humidity dependence, high ionic contamination and limited lifespan are largely eliminated.
Other technologies are currently under development which will undoubtedly yield solutions for plastics that are clean, ESD-safe and economical. These new materials will help pave the way for next-generation consumables together with yield and integrity improvements.n
Rob Linke is director of marketing for the Texwipe Company (Upper Saddle River, NJ), where he manages industrial product development and marketing. He holds a BSME and a BA in economics from Tufts University. His previous experience includes vertical transportation systems, automated board testing, strategic facilities planning and software development tools. He has been responsible for the development of the swab and cleanroom stationery lines at Texwipe and is a senior member of the Institute of Environmental Sciences where he is vice chairman of the working group for swab testing (RP-025). Linke is also an active member of the IDEMA and The EOS/ESD Society.
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A surface resistivity probe is being used to examine the inherent resistivity of a substance. Samples need to be “conditioned” at a fixed humidity level over a given time period to allow for consistent measurements.
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Carbon particles released from carbon-filament polyester textile.