Air ionization: Theory, use and best practices
By Arnold Steinman, MSEE
Ion Systems Inc.
Air ionization is an important part of comprehensive static control programs, and there is a reason that ionization is used for static charge control — it works.
Following are several air ionization success stories, each representing a significant cost savings to the manufacturer involved. The cost of the ionization was a small fraction of the value of the product or production time saved in each case.
Users of ionization systems in wafer fabs have reported reductions of 50 percent to 90 percent in the number of particles that are attracted to semiconductor wafers.
A manufacturer of plastic film found that charges of 20,000 to 40,000 volts were reduced below 500 volts after ionization was installed in the cleanroom. Contamination was reported to be reduced by 80 percent.
A disk drive assembly using MR heads is not practical without a comprehensive static control program including ionization.
Yields for disk media production are 5 percent to 10 percent higher with ionization in use.
Additional benefits that users of ionization have found include:
Improved yields in the testing of ICs.
Less frequent microprocessor downtime on robotics and equipment of all types.
Improved cleaning and fewer rejects of medical devices.
Let`s look at some examples of the use of ionizers in the cleanroom industry. The goal is to develop a sense of the “best practices” in using ionizers. Many of the following examples are in the advanced processing tools of the semiconductor industry. These techniques are applicable to electronics, disk drives, flat panel displays, medical devices and many other industries as well. There is a clear trend toward automated manufacturing, no matter what product is being produced.
Early cleanroom designs were basically one or a series of large rooms containing all the processing equipment. This was known as a ballroom design. This eventually evolved into the more modern “clean tunnel” which grouped all the parts of a single process, such as photolithography, in one area. The approach to using ionizers in these environments was generally total room ionization.
The trend is now to enclose the equipment in its own “minienvironment” and isolate it from the particle generating personnel of the cleanroom. In addition, more of the production process is carried out completely within the equipment, the cleanroom serving only for the transport of materials.
The type of ionizer needed in these point-of-use applications is different than those used for room ionization.
Ballroom ionization
In Class 100 or better cleanrooms, the ceiling is typically 100 percent filters. Ionizers mounted at ceiling level use the laminar airflow to deliver ionization to the work surfaces in the room. Room ionization is used when it is not possible to localize the static problem to one particular location or process.
Minienvironment ionization
In cleanrooms using minienvironments, room ionization is blocked from entering the work area just as are the airborne aerosol particles. Special attention must be paid to minienvironments to insure that ionization is delivered to the product.
The choice of ionizer type will depend on the size and height of a minienvironment. But typically, ceiling or bar-type ionizers are used. Ceiling ionizers are used at the top of a minienvironment, just as they are used in a cleanroom. Pulsed DC ionizing bars are used within the enclosure.
In the tightest spacing, particularly inside of process tools, compressed gas delivery systems are sometimes required.
Photolithography ESD problem:
Reticles
In photolithography processing, the effects of static charge on reticles are of particular importance. Reticles are the “negatives” used to print patterns on the wafer surface. Since step and repeat techniques are now commonly used to create multiple dies on the wafer, a single defect on a reticle will cause many defects on a wafer. Inspection techniques are only partially successful in preventing this. The photographic methods of the semiconductor industry are similar to those of other industries as well.
Reticle construction encourages the creation of static charge. The substrate is quartz, an insulator, on which the patterns for the chip are created in conductive chrome lines. An insulating cover, or “pellicle,” is placed over the chrome lines to exclude particles from the chrome surface.
The quartz substrates of reticles are good insulators, capable of accumulating high static charges, which attract particles. Electrostatic discharge (ESD) is known to damage the fine chrome lines, particularly at right-angle corners. Particles and ESD damage cause defects in printing the reticle image. Large numbers of defects can be created before a problem is identified. The result is either scrap or reprocessing time.
ESD damage to an MR head
ESD damage to MR heads occurs at very low levels of static charge. Typically, static levels must be maintained below 25 volts during the head assembly process. Failure to control static charge levels results in the type of damage where portions of the metallization of the head are melted.
Most important, ESD damage to MR heads has shown the latent defect phenomenon. ESD-damaged heads find their way into completed assemblies. Subsequent failures in final testing require disassembly and replacement. Failures of completed products cause catastrophic data loss at customer sites.
Preventing ESD problems in disk drive assembly is essential to achieving acceptable production yields. Ionizers used for this application are almost universally ionizing blowers that can achieve fast discharge times. The need for fast discharge is considered at this time more important than the turbulence created by the use of blowers in cleanrooms. Five-second discharge times from 1000 volts to less than 50 volts (sometimes to 10 volts) are considered acceptable for the current generation of MR heads.
All blowers used in disk drive assembly applications need to produce very low levels of particles or other contaminants. Testing should be performed to determine this. No blower is truly “compatible” with a cleanroom environment, but blowers that create acceptable low levels of air turbulence are available.
Contamination control
High-cost, high-efficiency air filtration keeps particles out of cleanrooms. Few particles come in through the air filtration system, but particles are still created within the cleanroom by personnel, production equipment, and the process itself. Unfortunately these particles are created in the worst place, right next to the product.
Laminar flow keeps particles entrained and away from product, but equipment surfaces and robotics interrupt the laminar flow. Cleaning critical surfaces often generates static charge.
As a result, we have particle production, no laminar flow, and high static charges in many areas of the cleanroom. The outcome is that particles are attracted to the product, causing defects.
Contamination in medical product manufacturing
The medical products area presents another opportunity to improve yields using ionization.
Medical products are generally exposed to the cleanroom ambient during most of their production steps. Plastic and glass are used extensively for their inertness and resistance to chemical attack. Unfortunately, these materials are also static charge generators.
Manufacturing with plastic and glass is not a clean process. Most manufacturing techniques used with these materials generate particles.
Sterilization is often needed for medical products. Liquid cleaning of surfaces with air or nitrogen blowoff results in high levels of static charge generation. Sterilization may require the product to be exposed for at least the cooling steps.
Particles on the medical product cause rejects when they are detected in the factory. They can be a much more serious problem when they are detected by or affect the customer during their intended use. Particles provide a medium for the transport of microbes, even if they are too small to be seen.
In order to remove charges from product surfaces to prevent particle attraction and prevent product handling problems, medical products manufacturers must implement a comprehensive static control program that includes room air ionization, bar ionizers and gas ionizers.
Room ionization with emitters at 6-foot centers is used throughout the production area. Additional bar ionizers are used over web fabrication processes and in cleaning areas to remove charges resulting from liquid cleaning and sterilization.
Compressed gas ionizers can be used for blowoff operations of both product and production equipment. All blowoff operations should be done carefully to avoid stirring up particles.
When implemented correctly, air ionization produces quantifiable results. For example, particle addition is reduced by as much as 80 percent during web processes; cleaning of critical plastic parts is more efficient; yield of medical optical devices is improved.
One example involved noticeable yield improvements in the production of IV bags. Visible particles were all but eliminated, and there were fewer returns and complaints by users.
Both manual and liquid cleaning processes are noticeably improved by performing the process in an ionized area. Keeping the static charge off surfaces prevents the reentrainment of particles that have just been removed.
Both implantable and contact lens yields have been improved by preventing the attraction of environmental particles during their production. Handling these small lenses by automated equipment is also simplified when they are not charged. Yields on other types of medical implants have been similarly improved.
Equipment ionization
The increased use of automation means that if ionizers are to solve static charge problems they must solve them inside the equipment itself.
There are many current applications for ionizers in electronics production tools. Among them are minienvironments, photolithography steppers, spin rinse driers, transfer systems and robotics, stockers/WIP storage cabinets, load/unload areas, optical inspection stations, test handlers, tape and reel component handling, pick and place equipment, and surface mount assembly and test. Ionizers have been installed to solve one or more of the problems associated with static charge, usually because there is no alternative technology to solve the problem.
In each case, the result is fewer damaged products and/or more efficient equipment operation. In some extreme cases, the equipment would simply not work without the use of ionizers to control static charge.
ESD problems in equipment
Static problems occur inside equipment for a variety of reasons.
All conductive and static dissipative materials in equipment should be connected to the ground. This prevents charge generation and accumulation and avoids the possibility of damaging ESD events. It is usually easy to ground stationary surfaces. Moving parts of equipment may present additional problems for reliable grounding.
Insulating materials may be a necessary part of equipment design, and it is generally impossible to prevent them from charging. Shielding techniques may be required to prevent their static charge from affecting sensitive products.
The most serious ESD problems in equipment, and the most difficult to solve, are generally caused by the products themselves becoming charged. The problem is most often connected to insulators that are part of the product and impossible to eliminate.
A comprehensive static control program is needed to solve the various equipment problems due to static charge. Some ionization approaches include air ionization and robots, bar ionizers in stepper minienvironments, controlling static in liquid cleaning applications, ionizers in cassette transfer tools, and gas ionizers in temperature chambers.
Air ionization has been successfully used around robots in a disk drive media manufacturing application, but the approach can be applied in almost any industry. Automation has brought the increased use of robots in many manufacturing processes. They can operate continuously with very low overhead costs. This advantage is lost if they require frequent attention or maintenance due to a static charge problem.
A robot in a disk drive manufacturing application was handling cassettes of coated glass disk media. When the problem it experienced was diagnosed as ESD-related, grounding methods were first used to prevent the static discharges from affecting the robot operation. It was soon recognized that this would provide only limited success, and that the static charge had to be eliminated at its source.
Installing ionizing bars over the robot was sufficient to neutralize the static charge on the product handled by the robot. Robot uptime increased more than twofold, and overall production yield increased by two percent.
The use of bar ionizers in a stepper minienvironment is another ionization approach. In this application, an ionizer bar was attached to the face of the ULPA filter located over the stepper.
Problems included malfunctions of the robotics handling both the reticles and wafers, and particle attraction to wafers, reticles and stepper optics. After ionization was installed, machine uptime increased with fewer robot “lockups,” fewer random particle defects were observed on wafers, and fewer repeating defects occurred due to particles on the optics and reticles. Weekly downtime for maintenance was reduced to once every 6 weeks. Overall product throughput increased by more than 10 percent.
Controlling static in liquid cleaning applications is another approach to ionization. In this application, very high static charges (over 20,000 volts) were noted on wafers and cassettes that were removed from spin rinser driers after processing. The static charge was generated by the contact and separation of the wafers and cassettes with the water used for cleaning. The use of nitrogen for drying created even higher static levels. This type of liquid cleaning is used on many other products manufactured in cleanrooms.
An air-ionizing chamber was connected in line with the nitrogen gas supplied to the inside of the spin rinser drier process chamber. Ionizers with exposed emitter points were not useable because of the grounded process chamber, the short distance to the product, and the presence of water and chemicals in the chamber.
The exposure time to the ionized nitrogen was sufficient to reduce static charge levels on the wafers and cassettes under 500 volts under all operating conditions. This was sufficient to minimize particle re-attraction and eliminate equipment problems.
Yet another use for ionizers is in cassette transfer tools. In this application, ESD events were interrupting the operation of cassette transfer equipment. The problem occurred only once every six to nine months, but the result was extremely costly — 25 semiconductor wafers dropped on the floor.
The problem was found to be static charge created during a cleaning process in a spin rinser drier. Sparks could be seen when the cassette transfer equipment attempted to remove the wafers and transfer them to the next process cassette.
The problem was solved by bathing the wafers and cassette with ionized compressed air for 15 seconds before the start of the transfer process. This was sufficient to reduce the static charge levels to under 100 volts, and prevent them from affecting the operation of the cassette transfer equipment. Of course, it would have been preferable to have effective static control in the spin rinser drier preceding the cassette transfer equipment.
Another method of air ionization, isolated ionization, can be used in temperature chambers and other areas within equipment. These areas are usually unsuitable for feedback control of the balance and stability of the ionizer. High temperatures and small dimensions are usually involved.
Isolated ionization makes use of the law of conservation of charge to maintain ionizer balance. The ionizer emitter points and their external power supply are both physically and electrically isolated from the ground. Conservation of charge requires that only equal numbers of positive and negative ions be produced. No feedback sensors and control systems are required.
Anyplace where cleanrooms are a necessity for achieving reasonable product yields, ionizers can assist in increasing those yields. Ionization is an important part of any successful static control program.
Arnold Steinman is chief technology officer at Ion Systems Inc., Berkeley, CA, responsible since 1983 for the design of the company`s ionization static control products. He holds four patents covering air ionizer technology. Steinman graduated from the Polytechnic Institute of Brooklyn, receiving a BSEE degree in 1965 and an MSEE degree in 1966. He is a member of the ESD Association and served as chairperson of the Ionization Standards Committee. He currently is the leader of the SEMI ESD task force, and serves on several other standards workgroups.
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A typical process area of a Class 1 cleanroom, which uses ionizers mounted at ceiling level to deliver ionization to the work surfaces in the cleanroom using laminar airflow (left). In minienvironments, ceiling ionizers can be used at the top of a minienvironment (below), or pulsed DC ionizing bars can be used within the enclosure (bottom).
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Ceiling ionizers are used to deliver ionization to the work area. This method is used when it is not possible to locate the static problem to one particular location or process.
This paper was presented at CleanRooms Europe `98. For more information on CleanRooms conferences and exhibitions, contact Nuala Kimball at (603) 891-9267. For a copy of the conference proceedings, which are available for $95, please fax your request to Libbey Duggar at (603) 891-9490 or call her at (603) 891-9462.