picture shows a silicon wafer
Dust-free environments are essential for the semiconductor industry, which uses delicate deposition and etching processes that repeatedly expose silicon wafers to plasmas.

Datelilne: April 2002.

Thanks to new tools that allowed researchers to create dusty plasmas in the laboratory for the first time, researchers are using this variation of the fourth state of matter to understand phenomena such as the melting of a three-dimensional solid and the propagation of sound waves in a two-dimensional dusty plasma crystal.

It was a frustrating problem. Somehow, despite extreme precautionary measures, semiconductor chips were being contaminated with dust and ruined in the process. Manufacturers looked long and hard at what could possibly be causing the difficulty. And when the semi-conductor industry discovered the culprit behind the mysteriously contaminated silicon wafers, researchers investigating dusty plasmas sat up and listened. What was vexing to the production of high-quality semiconductor chips allowed plasma researchers such as John Goree, of the University of Iowa, an opportunity to produce dusty plasmas in the laboratory for the first time.

Although the semiconductor industry is known for its efforts to provide absolutely dust-free “clean” environments for the manufacture of microchips, what was becomin#G·™ear was that while the usual sources of dust – operators’ skin cells flaking off, loose particles of dirt being kicked up by workers’ feet – were being adequately addressed, dust particles were settling onto microchips in some other way. The source of dust on the microchips was related to plasmas. Sometimes called the fourth state of matter, plasma is gas that has been ionized, which means an electric charge has released electrons from the nuclei of the atoms in the gas, leaving behind positively charged subatomic particles called ions, and unattached, freely moving electrons. The movement of the electrons in particular is greatly enhanced by the ionization process, as it acts to free the particles from their association with other subatomic particles in the nuclei of the atoms.

Naturally occurring plasmas are very hot gases, usually in excess of several thousand degrees Fahrenheit. Although they are less commonly found on Earth, they do make a rare appearance in the form of lightning. Also, the ionosphere, the layer just outside the Earth’s atmosphere, is a plasma. In fact, 99 percent of the universe is plasma. Cold matter like Earth is the exception, rather than the rule.

On Earth, the semiconductor industry uses artificial plasmas to deposit thin films onto silicon wafers or to remove layers of insulating or conducting metals from the wafers’ surfaces in a process called etching. The deposition and etching processes are how transistors and interconnections are made on computer chips, and they require exceptional control and an absolutely clean environment. The ionized gas that is used in this process, however, might actually increase the likelihood that a chip will be contaminated with dust.

When random dust particles from the air enter the gas, they too are ionized, allowing them to move freely and remain suspended, instead of settling out of the plasma cloud. This is what researchers like Goree call a dusty plasma. Once the electric field is turned off, dust particles that have grown large enough fall to the surface of the silicon wafers, contaminating them and making them unusable as semiconductors.

While finding dust suspended in their plasmas can be tremendously costly for semi-conductor manufacturers, their detection allowed Goree the needed setup to conduct experiments on dusty plasmas in a laboratory. Prior to the discovery of dust in the plasmas used in the semiconductor industry, no one knew how to make a dusty plasma in the laboratory, explains Goree: “The critical problem was what kind of instrument to use to make the dusty plasma. That’s when a semiconductor researcher discovered by accident a way of doing it.” Goree was involved with the field from the beginning, when the new kind of instruments began to be developed.

Add a Pinch of Dust

In his laboratory, Goree creates dusty plasmas using an apparatus with a design based on that of the equipment used in manufacturing semiconductors. “It’s a vacuum chamber with [gas and] two electrodes in it,” Goree describes. The electrodes are two flat metal plates. High-voltage electricity applied between the plates ionizes the gas in the chamber, result-ing in a glowing discharge of primarily neutral gas plus some electrons and ions. Dust particles (in this case polymer micro-spheres less than 10 microns in diameter) are put into something that’s analogous to a saltshaker. Goree explains, “We use polymer microspheres, but you could even use household dust if you wanted to.” The dust particles are shaken into the plasma, where they become electrically charged and will continue to be suspended in the plasma while electricity flows through the vacuum chamber.

Since the particles slosh around in the plasma, dusty plasmas are of interest to scientists working to understand the basic principles of fluid physics. Dusty plasmas are similar to colloids studied by fluid physics investigators examining the formation of ordered crystalline structures in suspensions of fine particles in liquid media. Like colloids, dusty plasmas consist of electrically charged particles of solid matter suspended in a fluid medium. While the medium for colloids in research investigations is typically water or water with added electrolytes, for dusty plasmas, the medium is a gas, such as oxygen.

Colloidal suspensions and dusty plasmas exhibit many of the same properties. For example, the particles of solid matter in a colloid are also electrically charged, so they repel one another, maintaining a certain distance between neighboring particles. The way that they achieve and maintain this separation is by arranging themselves at equal, fixed intervals in an array. The more the particles shift about with respect to their nearest neighbor, the more liquid-like the structure is said to be. More stable arrays, in which the particles move relatively little with respect to their nearest neighbors, form lattices or colloidal crystals. “Both the liquid-like and crystalline states can be achieved in dusty plasmas,” says Goree, “but we can also achieve another state that is gas-like, where the particles fly freely past one another and don’t ever stay near the same nearest neighbor.”

What’s also useful about dusty plasmas is that even in the solid or crystalline state, the movement of the particles in the plasma is less diminished by the surrounding medium than the movement of particles in a colloidal crystal. In a colloid suspension, the particles have to push around a lot of heavy liquid in order to move, losing a great deal of energy in the process. Particles in a dusty plasma can move about in the much less dense gas with less loss of energy. This diminished damping makes dusty plasmas particularly useful for observing phenomena such as the propagation of waves in a fluid. Goree explains, “As you speak, your voice is causing air molecules to be [repeatedly] compressed and then rarefied [stretched and thinned]. In a dusty plasma, because there is very little damping, the sound wave propagates very easily,” he says, making the propagation of sound waves much easier to study.

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Dark black dots can be seen forming a regular hexagonal lattice pattern as each dust particle in this dusty plasma crystal is surrounded by six neighboring particles. The particles in dusty plasmas all repel one another because of their negative charges.

Goree is using the unique properties of dusty plasmas in a number of ways. His ground-based laboratory experiments have two focuses. The first is the melting of a two-dimensional dusty plasma crystal in which the particles have arranged themselves into a crystalline lattice. Adjusting the strength of the electric field applied to the plasma (and thus the repelling force of the charged particles) allows the particles in the dusty plasma to move so that they are no longer permanently adjacent to the same nearest neighbor. The particles become disordered as the lattice melts into a liquid phase. This process is called “two-dimensional” melting because gravity causes the particles to sediment into a horizontal, almost two-dimensional layer.

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Sound waves near a loudspeaker cause air particles in their path to move in a repeating pattern. Goree’s research uses laser light to move solid particles, less than 10 microns in diameter, in a plasma crystal. Shown here, the resulting movement of the particles creates a pattern of sound waves similar to the Mach cone from a supersonic object in air.

The second focus of Goree’s ground-based effort is studying the propagation of sound waves in a two-dimensional dusty plasma crystal. In this experiment, Goree uses laser beams to stimulate the propagation of the sound waves. The laser beams push the particles in the dusty plasma crystal in the same way that air molecules next to the cone of a loudspeaker are pushed by the sound coming out of the speaker.

“You might not think that light can apply a force to an object,” Goree notes. “For example, if you were to take a laser pointer and point it at your hand, you wouldn’t feel your hand being pushed away from the laser pointer, but it actually is applying a force to your hand. If you were to take that same laser pointer and point it at a particle that is smaller than 10 microns in size and is freely suspended, that particle would begin to move. Then if you chop that laser beam on and off, the particle will move forward and cease moving [correspondingly]. In that way, you can make an alternating or cyclical motion in the particle that is like the alternating or cyclical motion of the air molecule next to a loudspeaker.”

Goree also receives funding from NASA to sponsor his work as a co-investigator with German Principal Investigator Gregory Morfill, of the Max-Planck Institute, on a flight experiment designed to study melting in three-dimensional dusty plasmas in the low-gravity environment aboard the Inter-national Space Station (ISS). With the reduction of the forces of gravity that cause the particles in the dusty plasma to fall as sediment in ground-based laboratories, particles in the dusty plasma experiments conducted in space can fill a three-dimensional volume.

“Melting a three-dimensional solid is very different, microscopically, from melt-ing a two-dimensional solid,” says Goree. “In space, we can study the melting process, observing microscopically where all of the particles are and how they move about with respect to their nearest neighbors, and we can do it not in two dimensions, which is kind of a synthetic thing under Earth conditions, but in three dimensions.” This ability gives researchers a better under-standing of the melting process in three-dimensional solid matter.

The flight investigation, called the Plasmakristall Experiment (PKE), was launched aboard a Russian Progress rocket on February 26, 2001. It is funded primarily by the German space agency, the DLR, and is conducted in cooperation with the High Energy Density Research Center in Moscow, Russia. Data from the experiment, the first to be performed on the ISS, are obtained through the use of video recordings of the melting process that are returned to Earth after resupply missions to the ISS. So far, some 40 hours of experiments have been run.

New Tools Foster New Discoveries

Although astronomers have long known about dusty plasmas in interstellar space, the ability to make dusty plasmas in the laboratory has proven key to making new discoveries. Early results from the ground-based and flight-based experiments are stimulating the development of new theories to describe the melting process in dusty plasmas and the subsequent phase change that takes place in the material. “In fact,” says Goree, “the field of dusty plasmas has been experiencing a very rapid growth in publications in the past 10 years.”

Dusty plasma research may also get a big boost from a long-term facility that’s being designed for conducting dusty plasma experiments aboard the space station. The International Microgravity Plasma Facility has passed the feasibility study, which assesses many practical details of the design, operation, and use of the facility given certain space station capabilities and restrictions. The project is led by the DLR, which is seeking partners to share in both the resources required to build the facility and in the use of the facility for research. “The facility would have many of the same kinds of components built for PKE, but unlike PKE, it is designed to be reusable and adaptable,” Goree explains.

Goree is looking forward to the opportunities opened up for him by the ISS. He sums up the significance of new tools and facilities to conduct research and the excitement of discovery: “When you have a new kind of experimental tool, it allows you to make new kinds of experiments or observations that were previously impossible to do. That stimulates a whole scientific community to grow. You get lots of new kinds of phenomena that were previously unobserved. It all just snowballs.”

Related Reading

For more information on Goree’s research, see: