When it comes to handling static-sensitive parts, controlling static discharge is not an option—it is a necessity. Small device geometries make semiconductor parts particularly sensitive to ESD, especially so because they could fail at voltages as low as 25 V. And a charged wafer or die can attract particles that contaminate the materials or form conductive paths and cause failure. So whether the work is being done at a bench, on an assembly line, or with automated equipment, static charge must be eliminated.
Don’t develop a false sense of security because you’re following most of the traditional static-control measures. Static-related part failures and process problems still can occur. Since many parts of the equipment in your process contain rubber, plastic, or other insulating materials, charge can’t be drained from them because they don’t conduct, and grounding them has no effect. For more complete control, you must remove static charge from insulators.
You can do this by coating the insulators with something more conductive or using ionization. There are times when a combination of these remedies works well and other cases when one solution is preferred. Coating the insulators may be impossible because of lack of access or if insulating properties are required for the application. Ionization may be the only practical solution.
Understanding Ionizers
To get the full benefit of ionization, you need a good understanding of the characteristics of ionizers, such as neutralization range, size, airflow, and temperature. With this information, you can see how ionizers control static in your process. Here are some different ionizer types and their associated properties.
Nuclear and Photon Ionizers
The nuclear ionizer is different from other types because it doesn’t require electricity and can be used in explosive environments. The nuclear material is a balanced source of positive and negative alpha ions. No airflow is necessary, and no EMI or electric fields are generated. The ionizer can be miniaturized and incorporated into equipment.
Maintenance on nuclear ionizers also is fairly simple, according to Richard Rodrigo, engineering manager at Simco. You do not have to deal with deposit buildup, he said, but the active element must be returned to the manufacturer on a yearly basis for testing and refurbishing.
All other commercial ionizers require electrical power, and many include high-voltage sources. They cannot be used in explosive environments. The photon ionizer uses a photon tube to emit ultraviolet light. Equal amounts of positive and negative ions are emitted, and balance calibration is not required.
No contamination particles are generated, making the photon ionizer a good choice for clean rooms. There are no emitters to clean. It can be used for ionization within equipment, but the target must be within the line of sight, up to about 1 meter. Photon tube life is about one year.
Corona Ionizers
Many commercial ionizers operate on the corona ionization principle. By concentrating a high-voltage electric field at a point, the dielectric breakdown voltage of the air is exceeded. High voltage is applied to an emitter point, causing a corona effect which ionizes the air. Corona ionizers have the following in common:
• Designed to be balanced or to emit an equal number of positive and
negative ions.
• Contain high-voltage emitter points which must be cleaned approximately
monthly and replaced about once a year.
• Create ozone which is limited by law to 0.02 ppm.
• Have emitters that may give off particles to contaminate a clean room. Some
germanium and silicon emitters give off fewer particles.
A corona ionizer’s behavior is determined largely by the type of voltage applied to its emitter points. In the AC ionizer, for example, high-voltage AC exists at the emitters. Positive ions are given off during the positive half cycle, and negative ions are emitted during the negative half cycle. High airflow, usually provided by a fan, is added to prevent positive and negative ions from recombining.
AC ionizers generally are simple and reliable and have low emitter maintenance. Typical applications are air guns or benchtop units. Though inherently self-balanced because of equal positive and negative half cycles, they are not easy to adjust once the balance is offset.
The steady-state DC ionizer consists of half the emitters at a high positive voltage and the other half at an equally high negative voltage. Due to emitter separation, high airflow isn’t needed to prevent ion recombination. They can be used in air guns, ionizing bars, and laminar flow hoods or with variable-speed fans.
Steady-state DC ionizers have low particle emissions, generate little EMI, and are easy to miniaturize for machine applications. All DC ionizers should incorporate circuitry to shut down if either polarity voltage fails since this would cause only one polarity of ions to be emitted and seriously imbalance the output.
A modification of the steady-state DC ionizer handles long-range applications. Pulsed DC ionizers are used for a distance of 1 to 5 meters and sequential bipolar DC ionizers are usable beyond 5 meters.
Instead of having both polarity voltages present simultaneously, the pulsed DC unit alternately applies a positive voltage to half of the emitters and a negative voltage to the other half. Each polarity of ions is repelled from its respective emitter before the other polarity appears. This practically eliminates recombination, and natural ion-repulsion force is sufficient to allow operation with or without air.
Sequential bipolar DC ionizers alternate positive and negative voltages as well, but they control the duty cycle of each polarity. The added off-time improves natural ion repulsion and increases the operating range. Balance is adjustable.
Matching Ionizers to a Process
In the past, it was difficult to ionize some environments that really needed it, such as inside process equipment and small enclosures, according to Steve Linsley, technical support manager at Ion Systems. The lack of space to mount the ionizers, the proximity to sensitive product, and the need to minimize airflow were among the challenges.
Of the ionizers used in process equipment, some have drawbacks. For example, compressed gas ionizers need a gas supply, and nuclear ionizers require regulatory paperwork.
Reducing air velocity is important, according to Brad Jubin of Semtronics. In some applications, such as giant magnetoresistive (GMR) head production, high air velocity can blow the heads right off the bench. Many ionizers, he said, now incorporate smaller fans to reduce the turbulence and overall air velocity in the work area.
Photon and corona ionizers have been developed that operate with little or no airflow, said Mr. Linsley of Ion Systems. As a result, high ion output units can be used with less airflow.
Creating More Bench Space
Anything that will create more room on the benchtop always is welcome, and ionizers are no exception. Workstation real estate is a primary concern, according to Bob West, a design engineer at ESD Systems. Today, many ionizers are small enough to be mounted on a swing arm above the bench, much like a lamp. This frees up workspace and allows you to focus the ionizer where you need it, he explained.
Ionizers also are making more efficient use of clean-room real estate, according to Harold Schweriner, marketing vice president at ElectroStatics. By using the laminar air supply in ceilings or hoods and HEPA filters on benchtops, these units don’t require a built-in fan. Using this design, tabletop ionizers take up much less space, and, of course, an overhead ionizer occupies none.
Conclusion
The semiconductor and recording head industries now use ionizers adapted for wafer handlers and wafer-cassette transfer equipment. These industries are controlling ESD with ionization techniques that are being applied successfully.
Leading-edge ionization applications such as these may translate into an answer for your process, or it’s possible that an off-the shelf solution already exists. A look at the products that follow may help your decision-making.
References
1. Steinman, A. and Levit, L., “Ionization for Production Equipment,” EE-Evaluation Engineering, April 1997, pp. 48-55.
NOTE: This article can be accessed on EE’s TestSite at www.evaluationengineering.com. Select EE Article Archives and use the key word search.
2. “Ionization in the Semiconductor Industry,” Simco Handbook, 1997.
Ionizers
Self-Balancing Ionizer
The VSE 3000 Volume Static Eliminator is a workstation ionizer that is self-balancing to 0 V ±5 V. When measured according to EOS/ESD Standard 3.1, it has a decay rate of 3.8 s at 4 ft. The unit includes a quiet, variable-speed fan and a separately switched heater. Stainless steel emitter points are easy to remove for cleaning or replacement and rated at 50,000 hour life. The tilt stand is removable so the VSE 3000 can be placed on a benchtop or a wall. $425. Chapman, (207) 773-4726.
Rapid-Decay Ionizer
The IT-7100 Table Top Ionizing Transport System uses DC technology to produce three times the amount of positive and negative ions as AC systems. The variable-speed air system permits a gentle flow of balanced ions that are effective up to six feet. It has an offset voltage <5 V and a decay rate from 1,000 V to 100 V in 0.8 s when measured per EOS/ESD Standard 3.1. A fail-safe balance indicator assures proper ionization output. $435. ElectroStatics, (215) 513-0850.Compact Ionizer
The Eliminator Jr. H/O Ionizer is a compact, lightweight, benchtop unit that uses steady-state DC technology. An active-feedback circuit maintains ion balance despite variations in line voltage, air speed, or emitter-electrode condition. It eliminates the need for balance calibration and gives a visual indication when maintenance is required. A two-speed fan and an air filter are included. Models are available with or without a heater. An optional mounting arm allows off-bench placement. $302.33; with heater: $392. ESD Systems, (508) 485-7390.
Ionizing Bar
The Ionizing Bar removes static electricity from plastics, webs, or sheet stock. It is compact for mounting in confined spaces or machinery where charge is generated. A high concentration of positive and negative ions produces rapid static decay. For long life, the bar has stainless steel emitters which run at 5 kV. Also, there are no openings to admit dirt that can cause malfunction or sparking. An optional Super Air Knife attachment increases the static decay rate and provides wide area coverage. From $ 140. Exair, (513) 671-3322.
Process Ionizer
The Model 4630 QuadBar™ Ionizer can control static charge in 15°C to 50°C ambient air inside a process chamber. It fits into the tight confines of process equipment, and it can be powered from the 24 VAC source of a machine tool. Up to four units may be daisy-chained for greater coverage. The unit conserves charge to maintain a ±50-V balance, and no calibration is required. Replaceable silicon emitters produce few particles, resulting in clean ionization. Call for price. Ion Systems, (510) 548-3640.
Zero-Volt Ionizer
The ZVI5000™ Zero Volt Ionizer™ is a compact, single-fan, benchtop ionizer that uses steady-state DC technology. It has low air velocity with high area coverage and a rapid decay time. Feedback from a circuit sensor provides data on balance control, and audio/visual signals indicate when balance is achieved or maintenance is required. The ionizer is easy to disassemble for cleaning and maintenance. $445. Semtronics, (770) 487-6700.
Ionization System
The Gemini™ Low-Voltage Ionization System consists of emitter modules interconnectable via RJ-45 telephone jacks. A controller, powered by 90 to 240 VAC, 50/60 Hz, supplies up to 100 emitter modules with +28 VDC. The controller also communicates with the emitter modules on an RS-485 multidrop network. To adjust an emitter’s output and balance individually, a hand-held remote is available. Emitter operating modes include 0 to 20 kVDC for each polarity with steady-state DC or pulsed DC drive. Replaceable emitters are tungsten or germanium. Call for price. Simco, (800) 538-0750.
Copyright 1999 Nelson Publishing Inc.
April 1999