Terminating static in the cleanroom
Static charge generation is unavoidable and will never be eliminated, but you can minimize its damaging effects by following three basic steps.
By Sheila Galatowitsch
You need only look to Hollywood to find evidence of a growing awareness of static charge contamination. In the hit movie Independence Day, military officials attempt to restrict the president from entering a research facility by warning: “It`s a cleanroom. We have to keep it static-free.”
Electrostatic discharge (ESD), together with its offspring electrostatic attraction (ESA) and electromagnetic interference (EMI), can all contaminate medical devices, pharmaceuticals and electronics. ESD can vaporize metal lines on semiconductors, melt silicon and cause device failures; damage masks, reticles, wafers and disk drive heads; cause random defects on flat panel displays; and produce equipment failures.
While awareness may be increasing, experts claim that knowledge of how to combat it is still limited. Static electricity “is considered a nuisance,” says David E. Swenson, president of the Electrostatic Discharge Association (Rome, NY). “People have other fundamental problems, like dealing with the end-product yield, even though static electricity may be contributing to the problem.”
“Many people are still in denial,” says Steve Nosek, senior test equipment and certified electrostatic discharge control engineer with Motorola Semiconductor Products Sector (Chandler, AZ). “They believe that ESD is not a problem in their areas.” Yet few industries are spared the effects of static electricity. Some, such as the disk drive industry where magneto-resistive (MR) heads are sensitive to as little as 5 volts, would simply not exist without drastic static control measures. With problems like these — resulting from a mostly unseen and unfelt force — a static control program is an essential insurance policy against loss.
Static charge, ESD and EMI
Static charge occurs when two surfaces come into contact with or rub against each other; from environmental conditions, such as exposure to light, heat and cooling; and from certain mobile processes, such as spinning, coating and plasma processes. Simple human movements can also generate charge.
At rest, static charge poses a potential contamination problem. Once a surface is charged, it will attract and hold small particles in the air having an opposite electrical charge. According to Arnold Steinman, chief technology officer at Ion Systems (Berkeley, CA), “even in the most stringent cleanrooms, static charge attracts particulates from people, processes and equipment.”
When static charge stored on one surface transfers to another, it results in an ESD event. Because most ESD events are indiscernible to the naked eye, cleanroom workers are not aware that ESD is occurring. The resulting product damage happens quickly and invisibly.
Moreover, ESD events can generate EMI. Although most equipment is designed to tolerate a certain amount of EMI, if it exceeds these limits, EMI can cause equipment errors. “In the best case,” says Steinman, “equipment will simply stop and be restarted by cleanroom workers. But in the worst case, a machine may make a costly mistake, like dropping a wafer.”
Although our ability to detect, measure and monitor static electricity is better today than a few years ago, says Swenson, “the ramifications of static electricity are also getting worse because of the increasing complexity of materials and parts.” As cleanroom users move to smaller geo metries, lower humidity levels, faster packaging techniques and greater use of insulating ma terials, the likelihood of static electricity damage greatly increases. “If you have a static phenomena occurring at a low level, it can cause problems that might not have been caused a few years ago,” adds Swenson.
Step-by-step problem-solving
Cleanroom users can control static in three steps: implementing proper grounding; replacing insulating materials when possible; and installing ionization technology where it makes sense. (In some geographic areas, low levels of humidity — lower than the 40 to 50 percent relative humidity of most cleanrooms — can also cause a static electricity problem. Adjusting the humidity may solve the problem in these cases.) These basic steps apply to cleanrooms in any industry, although each cleanroom setting will have its own unique grounding, materials and ionization requirements.
Each step must be taken sequentially. For example, if the problem is not solved by proper grounding, the materials must then be examined and replaced. If those steps fail to resolve a static electricity problem, ionization technology is the last and final solution. “Each method you put in solves a portion of the problem,” Steinman says.
Before implementing the steps, cleanroom managers should first conduct a static audit to verify that static charge exists and might be the cause of a problem, suggests Steinman. The audit involves bringing electrostatic field meters and EMI-locator instrumentation into the cleanroom to detect static charges and pick up signals generated by ESD events.
When static electricity is thought to decrease product quality or cause losses, users should try to identify the point or points where the defects and losses are occurring. For example, says Chris Janson, product manager at NRD Inc. (Grand Island, NY), “look at the path that the product is moving from entry to exit. At what point is the product open to contamination or ESD events that could cause damage? Yield will go up by moving through these steps.”
Ground, ground, ground
Once static has been identified as a problem, “the first and foremost requirement is to provide adequate electrical grounding for all conductive materials that might be in a facility, including people,” says Swenson. “That`s the hallmark of ESD protection — grounding everything that you can.” Grounding provides a path for static electricity to discharge safely, and in many cases, proper grounding solves the bulk of a problem.
Personnel grounding. The first line of defense is to ground people, says Ed Weggeland, vice president of Richmond Technology (Redlands, CA). If done properly, grounding will eliminate electrostatic charge build-up on human beings, he says.
Personnel at the workstation can be grounded via conductive wriststraps. Cleanroom users should avoid cloth and select metal, nonparticulating wriststraps. Wriststraps will not protect personnel who are mobile beyond the workstation, however. Mobile users should wear conductive heel grounders or footwear in conjunction with a conductive floor.
Both wriststraps and footwear must connect to the ground –wriststraps by plugging into a grounded connection and footwear through a conductive walking surface or floor. If a garment is groundable, it must connect to a person`s skin so that any charge will discharge first to the skin, then through the conductive footwear and floor, and safely to the ground.
Conductive flooring. Conductive shoes and conductive flooring go hand-in-hand. According to Weggeland, “a lot of people incorrectly assume that if they put conductive footwear on people (without having a conductive floor), the problem will be dealt with. They forget to think about the vehicle that actually conducts the charge away.” Likewise, a conductive floor without insulative shoes is equally ineffective.
Because it is difficult and expensive to retrofit a grounded floor, facility owners should include one when planning new construction. “On a new facility, the installation of a proper ESD-control floor does not add appreciably to the construction costs,” Swenson says. “On the other hand, if a cleanroom has an insulative floor and ESD becomes a problem, the choices are to either retrofit a conductive floor or put in total-area room ionization.”
Furniture and carts. Chairs, carts and shelving with rollers can generate static. “Many users incorrectly assume that black wheels on a chair, cart or shelf are conductive,” says Weggeland. “I recommend that even carts or chairs that are designed to be electrostatic-protective need to be grounded to a conductive floor by the use of drag chains. That`s a certainty in my mind. It provides an opportunity to visually verify a connection between the furniture and conductive floor.”
Drag chains are conductive metal chains that hook to the metal part of a cart or chair and then make contact with a conductive floor. Weggeland recommends a drag chain for each chair, cart and shelf.
Equipment. Because of their constant contact with and separation from other parts, automatic handling and robotic equipment are significant static charge threats. Users should ensure that all equipment is verifiably grounded.
In many cases, equipment parts are made from insulative materials that cannot be grounded. “If a machine is a combination of metal and plastic, deal with the metal by connecting to ground, and deal with the plastic by either changing it from an insulative to a static-dissipative material, or install ionization,” Weggeland suggests.
Most manufacturers certify that their grounding products meet ESD Association standards, but it is ultimately the end-users` responsibility to make sure that what they specified in a product is what they are getting. The ESD Association has developed standard test methods to evaluate wriststraps, footwear, floors, garments and other static control products. Also available from the ESD Association are recommended grounding practices.
Replace insulative materials
Grounding solves charge problems associated with people and non-grounded conductive materials, but it does not solve problems arising from insulative materials in the cleanroom. Insulative materials actually generate static charge, and in some cases, build up enormous charges. Users should replace insulating materials with conductive or static-dissipative materials wherever possible.
“Plastics and polymeric materials are frequently the culprits,” says Dr. Douglas Cooper, director of contamination control at The Texwipe Company (Upper Saddle River, NJ). These include the two most common sources of plastic in the cleanroom, polypropylene and polyethylene, as well as laminates such as Formica and glass.
Suspect insulators are apparel, including garments, shoes, gloves and headwear; wipers and swabs; any type of container, such as trays, totes, bins and carriers; dispensers; documentation materials; and packaging materials.
For example, while wet-wiping applications do not pose a problem, dry wiping with a non-static-dissipative wiper can create a local charge. Totes stacking wafers into bins can build up significant static charge. Gloves are also frequently insulators, and even though many garments may incorporate some elements of ESD control, they may in fact test as insulators.
“We are mainly talking about items on the workstation or in use at the workstation,” says Rob Linke, Texwipe`s director of marketing. “To be really safe, you don`t want any insulative material at all in the vicinity of the workstation.” This includes trays used to transport parts, swabs that are used to clean sensitive parts, and tweezers that touch the part. “Virtually all materials that come into direct contact with a product are available in ESD-safe varieties,” says Linke.
Identifying suspect insulators. Product failure is of course the first signal that ESD may be a problem. The traditional way of tracing failure to an ESD event is to physically examine the failed part for pitting, craters or melted areas using a high-power microscope. Once ESD has been identified as the culprit, however, determining which materials to replace, requires being something of a detective.
“You are not going to replace everything,” Cooper says. “You need to look at the path the product takes and see where along that path you first detect damage due to static discharge. Ask which of the possible culprits the product is likely to have come into contact with in the early part of the path.”
Alternative materials. Conductive and static-dissipative materials allow static electricity to discharge safely. Conductives are usually metals, and because they allow charge to transfer rapidly, they are best used when you need to get charge away from an area quickly. However, this sheer speed can also sometimes generate an ESD event, so it is best to use conductive materials in areas where there is no chance of the product being damaged.
The preferred replacement material is something between a conductive and insulative material. “These are called static-dissipative, and they allow the charge to drain, but at an intermediate rate — not too fast, not too slow,” Cooper says. Static-dissipative alternatives include materials embedded with conductors, such as metal or carbon, materials with topical and blooming antistatic agents, and inherently dissipative polymer alloys.
The choice of an alternative material will depend on the operation itself and the contamination requirements of the cleanroom. “In most cases what is added to materials to create some conductivity also has the possibility of getting loose and becoming a contaminant,” Cooper says. For example, the topical antistats that are sprayed or wiped onto surfaces, as well as the antistats that are mixed into plastics and then bloom onto the surface, may transfer off the surface. And, while carbon- and metal-loaded plastic is commonly used for cleanroom containers, many users are becoming concerned about the potential contam ination risks.
Plastic alloys are considered the safest alternative from a contamination and ESD standpoint, Linke says. The alloys are proprietary materials that inherently dissipate electric charges, but since they are available from only three or four manufacturers worldwide, they also tend to be a more expensive solution. Though many cleanroom users are moving to the new alloys, others continue to use and prefer the other alternatives. “Sometimes you are forced into a certain choice,” Linke says. “There`s no one miracle material that solves everyone`s problems.”
Choosing alternatives. A healthy skepticism is helpful when selecting alternative materials. Because some materials labeled static-dissipative don`t perform as advertised, verify that a material will perform to your specifications by conducting your own tests using known test methods. “It is extremely important to perform validated testing. It`s not that hard to do, but a lot of people skip that step,” Linke says. The most consistent way to evaluate materials is to test surface resistivity and volume resistivity, as described in the ESD Association`s test methods.
When replacement is not an option. In some cleanrooms, there is no way to avoid the use of insulative materials, especially when the products themselves are insulators. For example, “a flat panel display, basically a big sheet of glass, is the worst for static control,” Steinman says. Many medical devices and pharmaceuticals, as well as oxide-coated silicon semiconductors and some disk drive components also fall into this category.
The trend toward larger and larger components will exacerbate the problem. Says Steinman, “Wafers are going to 300 mm, and a glass panel, in five years, will be one-by-one meter–that`s a big chip with all insulating materials, and you can`t connect them to the ground.” Since these materials can`t be replaced, the next solution is to use ionization technology, a cleanroom-compatible method for controlling static charge on insulators.
Ionization equipment
Ionization technology works by making air (normally an excellent insulator) conductive. Ionization creates large quantities of positive and negative air ions that are attracted to the opposite polarity charge located on the product, equipment part or material surface. It effectively neutralizes the charge on surfaces of insulating materials, dissipating existing charge and limiting the potential of charge generation.
Kinds of ionizers. There are two kinds of ionizers — electrical and radioactive. Electrical ionizers generate air ions by intensifying an electrical field on a high-voltage emitter pin. The energy causes electrons to separate from the nucleus of the air molecules. Commonly referred to as “corona ionization,” it is available in three varieties: alternating current (AC), steady-state direct current (DC) and pulse DC.
AC applies high voltage to one or more emitters, with each emitter alternating between positive and negative polarities at supply-line frequencies. According to Jim Curtis, sales manager at SIMCO (Hatfield, PA), AC is typically applied to general industrial applications, but is also used in minienvironments or laminar flow benches. Steady-state DC has a minimum of two emitters, which are constantly producing both positive and negative ions. Steady-state DC is used in minienvironments and some whole-room systems. Pulse DC, the type most often used in cleanrooms, has one or more pairs of separate, alternating positive and negative emitters.
Radioactive ionizers use the isotope polonium to generate ions by high velocity emission of alpha particles. Commonly called “alpha energy,” it achieves ionization on an atomic level which creates an enormous amount of ionized air.
One key advantage of alpha-energy ionizers over corona is that they are always creating an equal number of positive and negative ions, providing an intrinsically balanced output. And, since alpha-energy ionizers don`t require electricity, they can also be easily installed inside gloveboxes, equipment or minienvironments as well as potentially explosive environments.
The chief disadvantage, however, is that the technology is regulated in the U.S. by the Nuclear Regulatory Commission. Because it relies on a radioactive isotope, the ionizers must be checked every 13 months to ensure that the polonium is totally encapsulated. That means users must track serial numbers and return units for new ones every year. This burden has limited the technology`s adoption, although it is used by various cleanroom industries from semiconductors to medical devices. NRD is the only company in the U.S. that manufactures alpha-energy ionizers.
Implementation: whole room vs. point-of-use. Ionization technology can be used to control static for an entire room, workstation or minienvironment. (Because of the nature of the technology, alpha energy is suitable only for point-of-use applications, while corona can be used for both.) According to Swenson, however, “whole-room ionization tends to be more useful for removing particulates from the air, while it is not able to deal with ESD at a bench. Then you have to use point-of-use ionization.”
For example, as described by Steinman, in some processes, users can pinpoint where static may be a problem, such as the MR head industry where heads can be exposed to ESD damage or at the workbench where products are being handled. “In these cases, whole-room emitters suspended from the ceiling would be too far away from the problem and not work as rapidly.”
In the semiconductor and flat panel display industries, however, it is often more difficult to localize the problem, Steinman says. Especially in photolithography areas where reticles can easily be damaged from ESD, ionization should be broad-based, with units installed on the ceiling and inside the equipment itself. “In areas where the consequences of ESD can be so serious, it makes sense to put in as much coverage as you possibly can,” Steinman says.
Product configurations. Ionizers come in various product configurations. For example, whole-room, bar-type ionizers rely on HEPA and ULPA filter airflow to drive ions down into the room. In most minienvironments, bars are mounted just below the filtration units so that the air flow carries the ions down to the target area. Bars are often placed over robot handlers and input/output ports.
Other configurations are available with their own air delivery systems such as overhead or tabletop units with motor-driven fans to control charge at workstations or workbenches. Handheld units are used to remove surface contamination or to dry devices. Blower ionizers are not used in Class 10 and Class 1 cleanrooms, however, because the motor and fan rotation might contribute to particle contamination. In these environments, bar-type ionizers should be installed at the workstation. Other products include nozzles, which can be used to ionize small environments where a bar might be too large, and air rings, which are similar to blower types but have no moving parts.
Though ionization is also offered as a option by many equipment manufacturers, according to SIMCO`s Curtis, “it`s still in a phase where it is gaining acceptance, and more machines are delivered without ionization than with it.”
Ultimately, though static charge contamination can be minimized, it is a naturally occurring physical phenomenon that will never be eliminated. However, by implementing a disciplined control program based on grounding, replacing insulative materials and installing ionization, cleanroom users can ultimately win the war against this destructive force. CR
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The flooring, furniture, equipment and people in a controlled environment can all be generators of ESD events. Photo courtesy of ION Systems.
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When grounding is not possible, ionization equipment can be utilized to control static for both whole rooms or point-of-use applications.
Photo courtesy of Simco.
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To help eliminate electrostatic charge build-up on humans, personnel at workstations can be grounded via conductive wriststraps.
Photo courtesy of Richmond Technology.
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Simple human movements can generate charge in a cleanroom, resulting in product damage quickly and invisibly. Photo courtesy of Richmond Technology.
ESD guide for cleanrooms due out in `99
The Electrostatic Discharge Association will publish guidelines for controlling electrostatic discharge (ESD) in cleanrooms no later than next year, according to Tom Albano, chairman of the ESD Association`s Cleanrooms and Clean Manufacturing Working Group. The guidelines will focus on designing a cleanroom facility with ESD and electrostatic attraction (ESA) considerations, and will identify equipment and processes that can be used in a static control program, bridging the gap between ESD and cleanroom technology.
As observed by ESD Association President David E. Swenson, the issues are complex. “There are a lot of material difficulties. In a lot of cases, there are parts that are mutually exclusive and you can`t solve the ESD problem because of the cleanroom issues.” For example, some typical ESD materials and equipment cannot be used in cleanrooms because of particle concerns, and some cleanroom applications do not allow ions to be generated. “You could do some nice things if you weren`t worried about particulates, outgassing, etc. Cleanroom requirements change the ESD game a lot,” says Swenson.
The ESD Association document will go beyond other published cleanroom guidelines that have touched on ESD issues but “didn`t go far enough,” says Albano. The working group will pinpoint “where problems exist and methods to prevent or design-out those problems.” A draft of the guidelines will be available for review at the association`s symposium to be held October 6-8, 1998, in Reno, NV.
Additionally, the ESD Association has approved the release and publication of five new, broader-based standards documents that cover electrostatic discharge and electrical overstress resulting during basic manufacturing processes. Three of the new documents are full standard test methods; the fourth is a draft standard test method; and the fifth is a draft standard practice.
The full standard test methods cover garments, seating and work surfaces. ESD STM2.1 – Garments provides a test method for measuring the electrical resistance of static control garments. ESD STM 12.1 – Seating-Resistive Characterization is designed to measure the electrical resistance of static control seating. ESD STM 4.1 (Revised) – ESD Protective Work Surfaces-Resistive Characterization, which updates a standard previously issued in 1990, provides test methods for measuring the electical resistance of static control worksurfaces. The new version establishes a recommended resistance for worksurface materials to groundable point of 1×109 ohms and a minimum resistance to groundable point of 1×106 ohms. The maximum resistance was changed to help ensure that a given charge from a test object in contact with the surface will dissipate in less than one second. The minimum resistance was added to help prevent device damage from a discharge that dissipates too rapidly through a more highly conductive worksurface.
Draft document ESD DSTM 13.0 – Soldering/Desoldering Tools provides test methods for measuring electrical overstress-related parameters on electric soldering and desoldering hand tools. Draft documents are developed for review and approval by the Association`s Standards Committee and Board of Directors. Drafts must complete an industry comment and review stage before being issued as full standards documents.
The final document, ESD SP 3.3- Periodic Verification of Air Ionizers, is a standard practice that provides a measurement technique to determine ion balance and charge neutralization time for ionizers in actual use locations at set intervals. — SG