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Berkeley Lab Develops Award-Winning Cold Plasma Source

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By Paul Preuss, August 4, 1997
BERKELEY, CA. -- The Constricted-Plasma Source, a rugged, clean, low-cost device with potential for applying thin films on everything from microchips to plastic wrap, has won a prestigious R&D 100 Award for its developers at the Ernest Orlando Lawrence Berkeley National Laboratory.

Plasma sources produce "activated" gases (ionized or electrically excited gases) which are widely used in industry to lay a thin layer of crystalline or amorphous film on an underlying surface. Such films include the semiconducting layers that make up integrated circuits and light-emitting diodes (LEDs); the transparent conductors that are used in flat-panel displays; and the coatings on "smart windows" that control heat and light from the sun.

"The beauty of our source is that it can run with virtually any gas," says physicist André Anders of Berkeley Lab's Accelerator and Fusion Research Division, who with Michael Rubin of the Environmental Energy Technology Division and Michael Dickinson of the Engineering Division developed the Constricted-Plasma Source (CPS). With nitrogen, nitrides such as gallium nitride can be formed to create blue light-emitting diodes and laser diodes, useful in manufacturing CD players, flat panel displays, and even traffic signals. With oxygen, the CPS can help make a variety of oxides including indium tin oxide, a promising "electrochromic" coating that allows the transparency of windows to be adjusted by hand; using the CPS, a process that now requires heating the glass to 400 degrees Celsius should be feasible at much lower temperatures, even near room temperature.

Existing plasma sources operate at low pressures and high energies. Common models that use a hot tungsten filament can react with some gases or contaminate the plasma; models that use microwaves or radio-frequency energy are clean but very expensive, selling for up to $80,000. Any source of high-energy plasmas can cause so-called ion damage in fragile crystal structures.

By contrast, Berkeley Lab's CPS can operate at either low or high pressures to produce cold (low energy) plasmas with "a higher percentage of the desired species" of ionized gas. Low ionic energies (typically only a few electron volts) permit the formation of delicate crystalline films impossible to make using other kinds of plasma sources.

Anders describes the unique design of the CPS as "beautifully primitive. There's nothing to break." The trick is to force gas and electric current through a narrow constriction -- a hole or series of holes in a plate between the source's negatively charged cathode and a positively charged anode. As pressure builds in the chamber behind the constriction, free electrons collide more frequently with the gas molecules, and bound electrons are stripped away more quickly, allowing ionization of the molecules at low temperatures. The result is a gas of charged molecules and atoms and free electrons -- a plasma. The electrons stream through the constriction, attracted toward the anode, which may be close to the constriction or far away behind the target, or may even be the target itself. Meanwhile the positively charged ions are attracted by the cathode and stay inside the chamber. The pressure builds until the less energetic ions are blown toward the target through the constriction, the only means of escape.

Cold is a relative term. The plasma produced by a CPS is called cold because its ions, atoms, and molecules are of comparatively low energy. Its electrons, however, are hot -- even a few electron volts translates to tens of thousands of degrees Kelvin -- but have little mass. Whether cold and heavy or hot and light, neither type of particle does harm to delicate substrates.

In his first attempt to build a CPS, Anders used a thin glass tube only a couple of inches long, which allowed him to observe the plasma inside it glow as electrons and gas molecules collided; since the little device wasn't cooled, its plastic fittings melted within five minutes, "but that was long enough to show me that it worked," he says.

The CPS now comes in many sizes and configurations. In a typical CPS, a fist-sized, stainless steel cylinder with fittings for gas, electricity, and cooling water constitutes the high-pressure chamber; the constriction is a single hole in a plate capping the cylinder. A CPS can also be built as a long, thin tube, in order to fit existing molecular beam epitaxy installations.

The constriction can also take the form of a wide slit -- or a straight row of holes, which works well for large-scale applications. Such a "quasi-linear" array about a foot long will soon be tested by a major window glass manufacturer; if it works well, it can be lengthened to as much as two meters.

A cold plasma source two meters wide would also be ideal for making, say, the kind of potato chip bags that are coated with shiny aluminum oxide inside to keep their contents fresh. The CPS can produce the needed enormous amounts of ionized oxygen at temperatures cool enough not to melt the wide plastic webs.

"Right now, glass coated with electrochromic or photochromic film is an expensive item," Anders says. "A luxury-car owner can afford a self-tinting windshield, but we'd also like to make variable-transparency glass cheap enough for a homeowner's windows."

Some of the CPS's possible uses are exotic, such as creating water-vapor plasmas for better ion-conducting hydrated films or implanting nitrogen ions in lithium, with the ultimate aim of making recyclable lithium batteries, a potential boon to the electric-car market. Berkeley Lab's Constricted-Plasma Source already promises cheaper, more reliable manufacture of a wide range of everyday products.

The Berkeley Lab's award is one of 36 won by Department of Energy laboratories. This year's awards bring DOE's cumulative total to 453. "These awards are proof of the excellent science and technology going on all the time at DOE laboratories," Secretary of Energy Federico F. Pe[[section]]a said. "This science is making a positive difference in people's lives. The variety of these innovations reflects the breadth of resources that the labs are using to solve practical problems."

The Berkeley Lab is a U.S. Department of Energy national laboratory located in Berkeley, California. It conducts unclassified research and is managed by the University of California.

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