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Particle Research Technology
by Steve Gigl
To most people, environmental engineering means developing non-polluting machines and industrial processes in everything from paper plants to automobile factories. The field, however, also encompasses the creation of special environments useful in industry. The University of Minnesota Particle Technology Laboratory is well-known and has provided industry with solutions to many particle-related problems. "We have research sponsored by both government agencies and industrial consortiums," says Professor and Director David Y. H. Pui. The nine faculty members and nearly 40 graduate students use the laboratory space, clean rooms, wind tunnel, and vacuum facilities to research areas as diverse as hard disk damage and particle beam mass spectrometers.
Each minute, we release around 500,000 particles into the air. Regents' Professor Benjamin Liu, the head of the microcontamination division of the Particle Technology lab, explains: "A person like you or I may release fibers from our clothes, or dust that had been resting on our skin." We see people through a cloud of their own dust, but the particles are too small to see even in such great numbers, so they are not a concern in our daily lives.
In the closed environment of the space shuttles, however, an abundance of such particles could cause problems. On Earth, the individual particles of our dust clouds slowly fall to the ground due to the force of gravity. In orbit aboard the space shuttle, however, the particles float in the air. If enough of the free-floating dust were to get into sensitive equipment carried and used by the astronauts, damage from these particles could threaten the missions. As a result, NASA became concerned that the space shuttles were becoming contaminated.
To help NASA decide if a contamination problem existed, the Particle Technology Lab designed an instrument package to be taken aboard the Space Shuttle Columbia on two separate occasions. Dr. Liu and his associates developed two battery-powered instruments, one to count particles in the air, and one to collect particles for later analysis. The counter uses a laser to detect particles in the air and activates every 15 minutes during a 10-day mission in order to collect 10,000 data points. The sampler consists of 4 stages; each stage corresponds to a specific particle size, and each size corresponds to an area of the body a particle is likely to reach. Anything larger than 100 microns doesn't get past the nasal region and is usually breathed out. Particles in the 10- to 100-micron range will not pass further than the throat area. Some irritation could occur, but it would be quickly cleared. Those particles in the 2.5- to 10-micron range tend to pass into the trachea and bronchi, but no further. Only particles smaller than 2.5 microns make it into the lungs and alveoli, the most sensitive part of the system. Airborne asbestos is dangerous partly because it is in this size range, which allows it to get into the lungs.
Two of the samplers were taken for each mission, so that 24-hour samples could be analyzed on the ground for the first day of each mission and a day halfway through. It was found that there was a far larger concentration of particles in the air than had been assumed when the shuttles were built, so NASA decided to install air filters in the shuttles to keep the atmosphere as clean as possible for the astronauts and the equipment.
Many industries also need controlled conditions for manufacturing. During the production of semiconductors, it is vital that as few contaminants as possible are allowed into the product. For this reason, these materials are manufactured by machines under "clean room" conditions to avoid flaws and contamination, which would lead to loss of yield. Gases used in the manufacture of semiconductors must be just as clean as the machines. To accomplish this, a "point-of-use" filter is used in the gas line. This type of filter draws gas in through a membrane from all directions and passes the filtered gas through the line. "When the company that makes this filter first developed it, back in the 1980s, they didn't know how good the filter was . . . so they came to us to do some testing," explains Dr. Liu. To test the filter, a system was developed in which particles of a certain size and material are created and passed through it. A particle counter capable of detecting concentrations of 1 part per billion is used to determine the number of particles the filter allowed to pass through. "So now we're trying to improve the system, to go down to maybe 0.1 parts per billion," says Dr. Liu.
Semiconductors can also be contaminated by particles while being manufactured into actual devices. Silicon dioxide, silicon nitride, aluminum oxide, and tungsten are all potential contaminants in the manufacturing stage for silicon semiconductor devices.
One way to detect these contaminants on a silicon wafer is through the use of a wafer surface scanner. Silicon wafers are very reflective, but when a particle rests on the surface of the wafer, it will scatter laser light, indicating its presence to the particles. This equipment, though, must be calibrated to properly detect the 500 to 3000 particles that may inhabit the surface of a 6-inch wafer. Normally, the wafer surface scanner is calibrated with wafers carrying polystyrene latex spheres. Since the real-world contaminating particles are neither spherical nor polystyrene latex, the development of realistic calibration methods for these scanners is desirable. Dr. Liu and his colleagues are working to understand how particles of uniform size but nonuniform shape affect the scattering of the laser light and the detection of particles. To do this, they deposit particles present in the manufacturing of the devices onto silicon wafers and put the wafers through a surface scan. The calibration of the equipment for more realistic particles allows for better detection of contaminants on the wafers. "We are probably the only place in the world that has enough experience to make good calibration wafers," says Dr. Liu, "so they are very interested in us continuing this research."
The Particle Technology Laboratory in the Department of Mechanical Engineering is dedicated to the study of minute particles and problems they can cause. By making itself a resource for industry, the Particle Technology Lab has helped its students gain experience and knowledge in their fields.
