Causes and mitigation of particles from air ionizers
Proper choice of emitter-point material and efficient operation allow ionizers to be used in the strictest environments.
By Arnold Steinman, M.S.E.E.
Static charge is a continuing cause of problems in semiconductor production facilities. It directly damages product and reticles through electrostatic discharge (ESD) events, interrupts equipment operation, and makes contamination control more difficult. In many of today`s world class semiconductor facilities, air ionizers are used to control static charge. However, there is still concern among potential ionizer users that particles generated either by ion emitters or by collection on the emitters, will result from ionizer operations. This article summarizes current information on both of these phenomena.
The uncontrolled transfer of static charge between two objects is commonly known as an electrostatic discharge, or “ESD event.” The energy contained in this discharge is sufficient to vaporize the metal lines on semiconductors and on the photolithography reticles used to produce them. It is sufficient to melt silicon and cause catastrophic device failures. High voltages resulting from static charge destroy semiconductor gates, whose dimensions continue to shrink with advancing technology. In the disk drive industry, ESD damages magnetoresistive heads, and it can be destructive to thin film devices in a variety of other industries.
In today`s high speed automated production environments, ESD has become an increasing problem. Profitability is as much tied to throughput — the amount of product moving through production — as it is to yield. When an ESD event occurs, it creates a significant amount of electrical noise and electromagnetic interference (EMI). With complex production equipment relying on microprocessors for control, noise and EMI often result in random stoppages of the equipment. Sometimes this requires a manual reset and only a small amount of production time may be lost. Production time losses can be more severe when the equipment that stops is, for example, a materials delivery system. In some cases, expensive product losses and scrap are the result, as well as equipment downtime to clean up or correct the problem caused by ESD. Finally, bad data generated by ESD events is sent to a factory management system. This can have a significant effect on throughput by changing the operating parameters for an entire factory.
The other common problem from static charge is the attraction and bonding of particles to critical surfaces. High levels of static charge increase the deposition rate of particles and make them difficult to remove once they are on a surface. The result is random defects on semiconductor wafers, disk drives, flat panel displays, medical devices, and anything else where the integrity of a thin coating is important.
The widespread use of ionizers, laminar flow hoods, and point-of-use applications has greatly improved the control of static charge in cleanroom environments, often increasing yields in some processes by as much as 8 percent to 10 percent. Despite the use of ionizers in many of today`s world class semiconductor production facilities, there is still a concern among potential users that particles will be generated by the ionizers and that these particles will be a hazard to the product.
Two issues are of concern: The first involves the generation of particles by the ionizer emitter points themselves. Ion emitter points erode over long periods of time, depending on the emitter point material and the level of the ionizing current. The second issue involves the collection of particles from the cleanroom environment on the emitter points. A white deposit forms on the emitter points after they have been in operation for varying periods of time. While methods of dealing with this deposit vary, it is clear that the material is the result of trace chemicals in the cleanroom air.
Particle emissions
Ionization has been used for decades to reduce losses caused by static charges. Until 1984, ionizer emitter points were commonly made of stainless steel. The phenomenon of particle emissions from ionizers was identified by Dr. B. Liu at the University of Minnesota in 19851. These emissions were later analyzed at Research Triangle Institute by Dr. R. Donovan2. These studies concluded that stainless steel emitter points produced large quantities of metallic particles (from 20,000 to 70,000 particles per cubic foot), and that the higher the ion emitter current, the more particles were produced. Donovan`s analysis also determined that most particles were in the size range of 0.003 to 0.03 microns. Shortly thereafter, tungsten and thoriated tungsten replaced stainless steel as the preferred ionizer emitter point materials. Coupled with the more efficient ion production of pulsed DC ionizers, these materials reduced particle levels to those acceptable for most Class 100 and some Class 10 areas.
There was a great deal of variability in the test methods used by these researchers. In 1989, Yost and Lieberman proposed a standardized test chamber for particle emission measurements from ionizers and other production equipment3. The purpose of the chamber was to isolate the ionizer from the uncontrolled cleanroom environment, but still simulate a cleanroom. The chamber was 2 ft ¥ 4 ft ¥ 6 ft (600 mm ¥ 1200 mm ¥ 1800 mm) with HEPA filters at each end. A fan at the input end produced 55 feet/min. (0.27 meter/sec) of airflow through the chamber to simulate cleanroom conditions. Background particle levels were less than 1 particle per hour. A key feature of the chamber was the six air jets on the front and back to ensure proper mixing.
The particle measuring instrumentation, a TSI Model 3760 condensation nucleus counter (CNC), was located within the chamber, 1 meter from the ionizer, to reduce measurement error. It had a 0.014-micron size threshold. This was important, since most particles created by ionizers are smaller than 0.03 microns. An optical particle counter with a 0.1-micron threshold, and temperature and humidity monitoring equipment were also used. A 20 picofarad plate and fieldmeter monitored ionizer operation. Details of the chamber are shown in Figure 1.
Thoriated tungsten and pure tungsten emitter points were proposed as a replacement for stainless steel in 1985. Unfortunately these materials had their drawbacks. While both had average particle emissions levels of about 100 to 200 particles per cu.ft. (size <0.03 microns), they produced episodic particle bursts at 10 times this level. To improve these materials, a titanium emitter point was introduced in 1988 reducing the average particle emission level to less than 25 particles per cu.ft. and eliminating the particle burst phenomenon. Today, titanium points are still the material of choice in most cleanroom industries, with the exception of semiconductor fabrication.
Semiconductor wafer fabs are concerned about metallic particles in any amount. Heavy metals, such as iron, chromium, or copper, diffuse rapidly into silicon, creating defects when they are processed into the silicon. These so-called “metallic killer particles” should be avoided. While titanium has a small diffusion coefficient into silicon, even the small number of titanium particles produced by ionizers was soon unacceptable. Non-metallic emitter points were introduced in 1992. Made from single crystal silicon, they produced less than 5 particles per cu.ft. Since the particles were not metallic, and were under 0.03 microns in size, they did not have the potential to become “killer particles.” Most ionizers in Class 1 semiconductor manufacturing use this silicon emitter point material.
Particle collection
Particle collection on ionizer emitter points is usually observed as a white crystalline material that appears to grow on the emitter tips in the area where the corona ionization process takes place. This deposit is generally referred to as a “fuzzball” or “white powder.” Particle collection on emitter points is likely due to gas-to-particle conversions and chemical reactions occurring in the corona region surrounding the ionizer emitter points. Analyses in a number of different research papers has indicated the presence of silicon, oxygen, sulfur, phosphorous, and boron on emitter points4. These elements, not surprisingly, mirror the composition of cleanroom filter media, silicone sealants in the media, and semiconductor process chemicals.
A number of theories have been advanced to explain the source of the material that collects on emitter points. One involves the shedding of small particles by cleanroom filters or the outgassing of the sealants used to attach filter media to the filter frame. Particle shedding of cleanroom filters has been linked to, among other things, trace quantities of the hydrofluoric acid used in semiconductor production. This same shedding is believed to be the cause of boron contamination in fabs. Outgassing of filter sealants is not desirable in semiconductor production for a variety of reasons and is being eliminated. By artificially increasing their levels, experimenters have positively identified sealant gases as a source of the material growing on the emitter tips5,6.
A second theory involves the presence of fugitive gases and chemicals from the manufacturing process. Analyses of the deposit material contain the same elements as process gases and chemicals. While deposits appear on all ion emitter points, in industries other than semiconductors, the rate is much lower, and the composition changes to primarily silicon and oxygen. A third theory, advanced by IBM researchers, involves some chemical interaction between ionizers and water vapor in the air, resulting in the formation of ammonium nitrate. Although in some way connected to the ambient humidity, the source of ammonium nitrate is more likely the large amount of ammonia evaporating from commonly used wet etching solutions, and from personnel. A recent paper by Balazs Analytical Laboratory (Sunnyvale, CA) identifies compounds of ammonium and nitrates as the most prevalent inorganic contaminants in cleanroom air7,8,9,10.
In looking for the cause of the problem, it was observed that particle collection was most prevalent in older Class 100 wafer fabs that had been upgraded to Class 10 or better. The problem existed, but at much lower levels, in newly built Class 10 or Class 1 facilities. Cleanroom design has changed in many ways during the last 10 years as cleanroom requirements increased from Class 100 to Class 1 and beyond. Filter media design has improved, reducing filter shedding. Teflon filter media has been proposed to provide resistance to acid damage. There are fewer indications of outgassing from uncured sealants in the media and silicone sealants are rarely used in newer semiconductor cleanrooms.
Ionizer maintenance
In advanced Class 1 wafer fabs, emitter points are typically cleaned at 3- to 6-month intervals. This is a minor maintenance issue, far less important than maintenance of other production equipment in the cleanroom. The white deposit is removed by wiping with a swab or cloth moistened with isopropyl alcohol. Older fabs (built before 1991) will need to pay more attention to the maintenance issue. Maintenance procedures may have to be performed as often as monthly. Each user of ionization will have to determine the appropriate maintenance program. Due to the small amount of erosion, emitter points should be checked annually, and replaced when necessary (usually every 2 to 3 years).
Conclusion
Every ionizer emitter point material produces some particles. The proper choice of emitter point material and efficient operation of the ionizer to reduce output current levels will allow ionizers to be used under the strictest requirements of semiconductor manufacturing. Non-metallic emitter points eliminate the concern about “killer particles.”
Air ionization has become the accepted method of controlling static charge in most advanced cleanroom areas. This acceptance would not have occurred in sub-Class 1 environments if there were still concerns about particle production by ionizers. Air ionization systems are currently available to meet the most stringent requirements of advanced semiconductor production, and are in use worldwide.
Arnold Steinman,M.S.E.E., joined Ion Systems in 1981 as chief technology officer after nine years as an electronics consultant specializing in digital, analog, and microcomputer design. He holds four patents in the field of air ionization.
References
1. “Characterization of Electronic Ionizers for Cleanrooms,” Liu, B.Y., et al., Proceedings of the 31st Annual Technical Meeting, Institute of Environmental Sciences (IES), 940 East NW Hwy. Mount Prospect, IL 60056.
2. “Polarity Dependence of Electrode Erosion Under DC Corona Discharge,” Donovan, R.P., et al., Microcontamination, May 1986, Canon Communications, 3340 Ocean Park Blvd., Santa Monica, CA 90405.
3. “Method For Measuring Particles From Air Ionization Equipment,” presented to the Institute of Environmental Sciences, Annual Technical Meeting, 1989, by Michael G. Yost and Al Lieberman.
4. “An Evaluation of Air Ionizers for Static Charge Reduction and Particle Emission,” Fehrenbach, D.M., and Tsao, R., Electrical Overstress/ Electrostatic Discharge Symposium Proceedings 1992, EOS/ESD Association, 7902 Turin Road, Rome, NY 13440.
5. “Formation of Silicon-Containing Particles by Corona Discharge Ionizer in Cleanroom Environments,” Kobayashi, et al., Proceedings of 12th ICCCS Conference, October 1994, Yokahama, Japan.
6. “Particle Formation of Materials Outgassed from Silicone Sealants by Corona Discharge Ionizers,” Namiki, et al., Journal of the Institute of Environmental Sciences, January 1996, IES, ibid.
7. “Targeting Gaseous Contaminants in Wafer Fabs: Fugitive Amines,” Kinkead, D. , and Higley, J., Microcontamination, June 1993, Canon Communications, ibid.
8. “Controlling a Killer: How to Win the War Against Gaseous Contaminants,” Kinkead, D. , CleanRooms, June 1993, PennWell Publishing, 10 Tara Blvd., 5th floor, Nashua, NH 03062.
9. “A Review of Advanced Wafer Cleaning,” Novak, R.E., presented at Contamination Control Strategies Conference, Austin, TX, February, 1994, Semiconductor Equipment and Materials International (SEMI), 805 East Middlefield Road, Mountain View, CA 94043.
10. “Chemically Clean Air: An Emerging Issue in the Fab Environment,” Mikulski, J., Semiconductor International, September 1966, Cahners Publishing Co., 1350 East Touhy Avenue, Des Plaines, IL 60018.
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Particle collection on emitter points is observed as a white crystalline material. It is likely due to gas-to-particle conversions and chemical reactions occurring in the corona region surrounding the ionizer emitter points.