University of Minnesota–Pioneering Particle Research
The University of Minnesota?s Particle Technology Laboratory in the university?s Institute of Technology has been working on small particle research for 40 years. In addition to developing many instruments and techniques for particle generation, measurement, sampling and analysis, the laboratory has also been working in fundamental aerosol research.
By Susan English
Established in 1982, the University of Minnesota`s microcontamination research program is a major source of research and expertise for the semiconductor and other industries. Faculty and students pioneer research on the theory of aerosol deposition on semiconductor wafers, the measurement of particle deposition, and the application of methods of computational fluid mechanics to cleanroom studies, thus paving the way for aerosol science and technology to be recognized and accepted as a legitimate and important area of study for the semiconductor industry.
The Particle Technology Laboratory in the university`s Institute of Technology has a long history of small particle research, dating back more than 40 years. Since its inception in the late 1950s, it has pioneered the development of many instruments and techniques for particle generation, measurement, sampling and analysis. Over the same 40-year period, the laboratory has also been engaged in fundamental aerosol research: the size distribution of airborne particles, mechanisms of particle generation, transport and deposition, and methods of airborne particle removal and control. However, the Particle Technology Laboratory is perhaps best known for the development of aerosol instrumentation and experimental techniques for aerosol studies. Many of these instruments and techniques are now widely used in various industrial applications around the world. In conjunction with the particle research program, the laboratory developed the theory of air and liquid filtration and methods for filter testing, supported by both government and industry. In fact, the laboratory`s program in filtration research is generally regarded as among the best in the world. Its work has greatly influenced the filtration industry in the acceptance of the modern filtration theory for industrial filter design and testing.
Research Areas
Today, the Particle Technology Laboratory`s program encompasses five major areas of research: cleanroom and microcontamination control; filtration; air quality and air pollution; respirable dust; and basic aerosol research and instrumentation. In addition to controlling microcontamination in cleanrooms and in semiconductor wafer production, particle research at the laboratory covers such diverse areas as the dust particles in mines and factories related to “Black Lung Disease,” which still afflicts substantial numbers of miners, and better filtration methods to protect soldiers engaged in biological warfare, most recently in the Gulf War, according to Dr. Benjamin Liu, Professor and Director of the Particle Technology Laboratory and Environmental Division. Dr. Liu, a professor of mechanical engineering has been affiliated with the university since 1956 and is the inventor/ developer of several widely used aerosol instruments. Until recently, Technical Vice President of Contamination Control at the Institute of Environmental Sciences (IES), he is well known for his basic particle research and microcontamination control studies in semiconductor and computer manufacturing.
As early as the 1940s, Liu says, research at the university began paving the way for the microelectronics revolution with its research on oxide cathodes in vacuum tubes, breaking new ground in surface analysis–which in turn opened the door to the development of semiconductors, solid state electronics, and magnetic film. Today, the university`s Microcontamination Research Consortium includes 14 member companies from the semiconductor and microelectronics industries, including IBM (Yorktown Heights, NY), Millipore Corp. (Bedford, MA), Samsung Electronics (Suwon, Korea), Texas Instruments (Dallas, TX) and Applied Materials and Particle Measuring Systems, Inc. (Boulder, CO). The Consortium`s objective is to support basic and applied research on microcontamination control. Students and faculty benefit from support of their programs, while the industry members gain a valuable R&D resource for solving industry-wide problems.
Facilities
The Laboratory has approximately 16,000 ft2 of general laboratory space, including a 10,000 ft2 cleanroom and an impressive collection of aerosol counters, analyzers and generators. Basic laboratory instrumentation includes precision microbalance, optical microscopes, gas chromatographs and analyzers, fluorometers, etc. Specialized laboratory facilities are equipped with a dust chamber, filter-testing facilities, an engine emission test facility, high purity gas supply systems, wind tunnels and vacuum systems. Microcontamination research instrumentation includes wafer surface scanners, high sensitivity air and liquid-borne particle counters, high sensitivity and high resolution mass spectrometers, as well as specialized apparatus for preparing wafer calibration standards and for advanced cleanroom garment testing.
Megasonic Cleaning
A team headed by Dr. David Kittelson is currently in the second year of a project on the megasonic cleaning of semiconductor wafers, funded chiefly by Sandia National Laboratories (Albuquerque, NM). In megasonic cleaning, wafers are first immersed in a cleaning solution, put into a tank, and then irradiated with very high frequency sound. Kitellson and his students found that bubbles created by the sound waves (cavitation) collapse very violently, producing a jet that hits the surface, somewhat like jet stream cleaning on a microscopic scale. Or bubbles may not collapse, but pulsate, abrading particles and helping to lift them from the surface of the wafer. In trying to understand the details of the sound field and how to model it, they found that unlike normal fluid mechanics, acoustic fields are very difficult to measure because sound waves don`t just go through something once: they reverberate in many different directions. This phenomenon, called “acoustic streaming,” occurs when a sound field is put through any medium, producing a beaming effect and ripples. The difficulty is in attempting to model the highly non-linear effect produced by acoustic streaming on a computer. Therefore, his team had to devise a special “pseudo 3-dimensional” model–really two dimensions with the tank going off in one direction into infinity,” says Kittelson.
Another phenomenon of sound transduced through water in the cleaning tank is “sonoluminescence,” or ultraviolet light emission from collapsing bubbles. When they collapse, there is enormous pressure, and the temperature at the center of the bubbles may go to extremely high levels–more than 10,000°F. These emissions, appearing as “glowing” liquid, can then be detected by a photo multiplier tube. The technique is being used to help the team understand where cavitation occurs in megasonic cleaning tanks, and ultimately, to attempt to link the appearance and intensity of cavitation with cleaning efficiency to produce a model that can predict how design changes in the cleaning process will affect cleaning efficiency.
Particle Beam Formation
Nucleation is the formation of molecular clusters from gas-phase molecules, which then grow to form stable particles. Prof. Peter McMurry found that with nucleation in semiconductor processing, a great deal of processing takes place in pressures substantially lower than the atmosphere, while aerosol instrumentation is typically designed to operate at or near atmospheric pressures. The particle beam mass spectrometer (PBMS), designed to size and detect ultrafine particles (-0.01 to 0.5 µm) in low-pressure environments such as those used in semiconductor processing equipment, has heralded the future for submicron structures on semiconductor wafers.
Says McMurry: “We`ve been able to measure size distributions and chemical vapor deposition systems used for poly silicon deposition and the deposition of oxides. So we`re now beginning to use the instrument to do fundamental studies of particle nucleation of these sorts of systems to see if we can develop models or semi-empirical theories to describe the particle nucleation growth processes that could occur. We`re able to detect particles perhaps as small as 5 µm with this method in pressures down to a few hundred milli-torr and lower.”
Gas and Liquid Filtration Media
Another cutting-edge technology is the fabrication of metal filters used in removing particles from high purity gas and liquid filtration systems in tool or production facilities. Traditional polymeric membranes have undesirable characteristics such as outgassing of hydrocarbons, oxygen, moisture, and plasticides. Now, stainless steel and nickel are the two predominant metals for use in critical processes, where the need is not just for gas that is free of particles, but also of molecular contaminants. Over the last 10 years, Dr. Kenneth Rubow has been studying high purity gas filtration, sampling systems (techniques for sampling particles in high pressure gas systems) and the filtration characteristics of filters used in high purity liquids, determining the efficiency of the material, and conducting evaluations on the fabricated products and gas line installations.
The aim is to minimize outgassing by minimizing the surface area within the filter. To do this, manufacturers are making filter media thinner and thus reducing the effective diameters of the filters that can outgas into the clean gas during processing. Dr. Rubow has developed experimental techniques to measure the extreme variations in concentrations between the upstream and downstream surface of filter materials. Currently, filters are being fabricated which claim particle removal of 9 orders of magnitude reduction in particle concentration for the upstream and downstream. Another concern is particle shedding, where the filter would actually be contributing particles instead of eliminating them, re-entraining particles into the gas stream as a result of the manufacturing process.
A second area is the problem of developing systems for sampling particles in clean gas systems. The problem is to reduce the gas which is entering at high pressure to atmospheric conditions so that traditional particle counters, condensation particle counters, or optical or laser particle counters can be used, without contributing any particles or losing any particles that are in the gas lines. After many years of research, Rubow and his team have developed special pressure-reducing devices, using orifices to drop 100 psi pressure down to ambient conditions.
In the area of filter characteristics of media used for high purity liquids, the dominant filter material is polymeric, mainly because of its inert characteristics. Dr. Rubow`s filter- testing system generates monodisperse polystyrene latex (PSL) particles in the 0.1 to 1 micron range, which are then suspended in high purity water and fed into the filter. Upstream and downstream concentrations are measured, just as with the gas filters. Dr. Rubow and his team found that the same mechanisms that allow particles to be captured in a gaseous filtration application, also act in liquids, although the magnitude of their effectiveness is very different. The particles are then mechanically “sieved” through a web-like structure made of material such as PTFE membrane, which manufacturers are continually trying to fabricate with smaller and smaller pore sizes. “Over the last few years, they`ve gone from 0.2 to 0.1 microns, and now we`re down into the .05 micron range,” says Rubow.
Bioaerosol Filtration
In conjunction with the University of Minnesota`s program on infection control, one researcher, Dr. Thomas H. Kuehn`s, areas of expertise is the filtration of bacterial and fungal spores in healthcare facilities, hospitals and pharmaceutical companies. As an HVAC engineer, Dr. Kuehn is concerned with filtering and maintaining adequate air flow and positive pressure relations not just in operating rooms, but in hospital recovery rooms and the normal hospital environs. n
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The University of Minnesota Particle Technology Laboratory has a 40-year history of performing small particle research. It has pioneered the development of many instruments and techniques for particle generation, measurement, sampling and analysis.
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A schematic diagram of a particle beam mass spectrometer (PBMS) used to size and detect ultrafine particles (-0.01 to 0.5 (m.) in low-pressure environments, such as those encountered in semiconductor equipment.
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Students inside one of the MLRE`s clean areas where semiconductor wafers are processed.
MLRE`s Cleanroom-within-a-Cleanroom
Besides the Particle Technology Laboratory`s own cleanroom, it uses the facilities of the Microelectronics Laboratory for Research and Education (MLRE), an independent microelectronic laboratory at the Institute of Technology housed in the electrical engineering department. The MLRE has its own industrial advisory committee consisting of members from area companies in the microelectronics field, and more recently, the biomedical segment of the community.
“It`s amazing how many big companies have cut back on their R&D and are looking for somebody else to do some of their work,” says Cleanroom Manager Dr. Greg Cibuzar, “but since we are supported by the State of Minnesota, we feel we need to make sure that our emphasis is in line with the university.”
Built in 1990 by Daw Technologies (Salt Lake City, UT), the MLRE contains a 3,000 ft2 Class 10 clean area. Additional support space brings the total to about 8,000 ft2. The cleanroom contains a completely outfitted area for semiconductor processing, including photoresist spinners, steppers, etching and CVD systems. The Particle Technology Laboratory has its equipment in Bay 3, where cleanroom garments are tested and particles are laid down on wafers. The more caustic experiments are conducted in a cleanroom inside a cleanroom–a containment system for possible leaks.
A bay-and-chase arrangement, the cleanroom has four 10 ft x 52 ft bays, each built on vibration-isolated slabs anchored in the underlying bedrock of the support structure. Each of the bays along its length is divided into seven zones, with its own transfer fan and cooling coil and RTE temperature sensor. This system ensures temperature uniformity at (68° F. and humidity at 38 percent (2 percent. A high purity 316 electropolished stainless steel distribution system runs cylinders in a bunker remotely so “house gases” can be accessed from any part of the lab. In addition, the lab contains a deionized water system, liquid nitrogen system and high purity nitrogen. Sound-dampening features have been installed for the convenience of classes and groups of people receiving instruction in the laboratory. The space is fully ULPA-filtered with airflow at 90 ft/min. and 20,000 cfm. There are three exhaust systems of welded stainless steel exhaust lines, separate gas cabin exhaust, scrubber exhaust and wet benches.
All hazardous and toxic gases have automatic purge assemblies that are hooked up to the building`s computer system, which in turn, is tied into the toxic gas monitoring system. In the event of a toxic gas leak, the monitoring system sends a signal to the building system, which sounds an alarm in the lab, and shuts down all the toxic gases at the cylinder near the automatic purge assembly.–SE
Designing Aerosol Instrumentation
Recent topics of research in filtration involve filter pleating design. A question being studied is: What is the optimal pleating design for a given geometry and for a given application? (Filters are pleated to reduce pressure drop and introduce more filtration area.) With his students, Dr. David Pui has developed a general correlation curve that manufacturers can use to design their filters. “Our laboratory is very well known, due to the fact that we have developed a lot of instrumentation which is used as the workhorse in application fields.” The faculty has several dozen patents that have been developed into commercial instruments widely used around the world, he says. For instance, a recent project was a feasibility study with the National Institue of Standards and Technology (NIST) to develop a method to certify the 0.1 micron Standard Reference Materials (SRM),–work that ultimately led to the NIST 0.1 micron latex sphere, able to be certified at 0.1007 micron with ۪.002 uncertainty.
A National Science Foundation grant supports the development of methodology for measuring, producing and charging nanometer particles. As chip feature sizes get smaller and smaller several years down the line, people will be looking at controlling nanometer-size particles, Pui says. The group has developed a technique to produce monodisperse particles in the 4 nm (0.004 micrometers) to 1.8 micrometer diameter range. The “electrospray aerosol generator” was designed to perform test calculations on nanometer particles, which are difficult to measure because their size causes them to behave “just like a gas,” he says. Because they diffuse very quickly, Pui says, “it`s tricky to measure these particles because they tend to get lost on the tubing wall. When you try to classify them, they spread out so much you think you`re measuring non-uniform-size particles, when actually the particles are diffusing in the measuring instrument.”–SE