By Marlene Cimons, National Science Foundation
In consumer products with flat panel screen displays—TV sets, cell phones, computers and GPS systems—speed is everything. The faster, the better.
These may become even faster in the near future, thanks to researchers at Oregon State University who have developed new technology with the potential to break existing speed barriers in a very big way.
And just as important, the materials involved in making them are environmentally friendly—non-toxic and non-polluting—and much less expensive than today’s silicon-driven components. The end result will be a faster, affordable device with little, if any, environmental footprint.
“We’re looking at switches that would work many, many times faster than what is currently available,” says Douglas Keszler, director of the Center for Green Materials Chemistry at Oregon State, and distinguished professor of chemistry, adding, “Researchers have been trying to do this for decades, until now without much success.”
The technology under development is called a tunneling diode, or MIM (for “metal-insulator-metal”) diode, which is a simple, thin film device, built like a sandwich, that uses special water-based inks created by center scientists. These inks, which contain small molecular scale inorganic chemicals, are printed onto different surfaces such as wafers and act as semi-conductors.
“We stack a metal, then a very thin oxide, then a metal on top of that, and we literally shoot electrons from one metal to another almost instantaneously,” Keszler says. “The fact that the electron transport occurs instantaneously means we can manufacture very high speed devices.”
Traditional silicon-based materials work differently, and more slowly. The silicon limits the flow of electrons in transistors, a process that restricts the speed at which electrons can move across a circuit, thus affecting how quickly computers and other devices can load programs.
MIM diodes, on the other hand, provide near-instant electron transfer, speeding up the operation many-fold. The new devices use a super smooth metal in thin film form, rather than aluminum, which is rougher, to control the flow of electrons and keep it even across the surface of the diode.
“The oxide—the middle layer—is something we produce with our inks, and, long term, we would produce the metals as well,” Keszler says.
The work recently appeared in the journal Advanced Materials, where the authors described the approach as “an intriguing new means both for designing very high-performance electronic devices and integrating them across multiple technology platforms.”
The researchers included Keszler and E. William Cowell III, Nasir Alimardani, Christopher C. Knutson, John F. Conley Jr., Brady J. Gibbons, and John F. Wager, all from Oregon State.
The MIM diode is one of several ongoing projects initiated by the center, a research facility created in fall 2008, with a three-year $1.5 million planning grant from the National Science Foundation. Its primary goal is to develop new electronic technology using a “green” inorganic chemistry approach.
Center scientists also are exploring the inks’ potential use in large area devices—an aircraft wing, for example—as well as making such objects as cell phones, garage door openers and personal computers even smaller than many already are.
“All will get smaller and faster and less expensive,” Keszler says. “We’re trying to make the process cleaner, with fewer steps and with higher performance.”
Currently, the manufacture of electronic devices is often wasteful and potentially hazardous, sometimes using dangerous or cancer-causing materials, and resulting in higher than necessary levels of greenhouse gas emissions, the latter a major contributor to global warming. In recent years, more and more scientists have been working to discover environmentally-gentle ways of making new consumer products, as well as cleaner, renewable energy sources.
Producing the MIM diode, for example, results in minimal waste, unlike the silicon wafer, Keszler says. With silicon, every gram of material in an integrated circuit produces about 1,000 grams of waste, he says. With the new diode, the inks are the active components, replacing those made of silicon.
“We’re coming at this from a completely different angle than what has been done conventionally,” Keszler says. “Ours is a green emphasis. But being green is only one driving force. Performance is another. We want the materials to perform better than what is currently available.”
The center’s early work has drawn upon resources within Oregon State, including its department of chemistry and school of electrical engineering and computer science, as well as the University of Oregon’s department of chemistry. “None of this happens without good chemistry,” Keszler says.
With the new diode, for example, the researchers purify plain tap water and add metal salts with constituents such as hydrogen peroxide—the liquid used medically to treat wounds—to produce specific “nanoclusters,” tiny synthetic compounds that serve as the basis for the inks. “We add these synthetic chemicals and mix them in controlled ways to produce the nanoclusters,” Keszler says.
“We stay away from any elements that are toxic or radioactive,” he adds. “We do a lot of things with zinc, and zinc oxide, aluminum, which is the third most abundant element in the ground around us, and iron, the main component of steel. A lot of these nanoclusters occur in natural water systems, and we are learning more about the behavior of these systems in natural water as well.
“When we print the ink, the water leaves, and the nanoclusters in the inks turn into very high quality films and patterns which function as semi-conductors, insulators, or metals,” he continues. “On almost all of these devices, you need a high-quality insulator that impedes the flow of electrons in order to temporarily store electron energy or act as a gate for controlling movement of electrons through an adjacent semiconductor. That’s extremely hard to do, and what our chemistry does very well.”
Through additional chemical modification, the usefulness of the inks can be extended by making them sensitive to light or electron beams, where they behave like the chemicals in photographic film, Keszler says.
“In this way, the light and beam sources for manufacturing integrated circuits can be used with the inks to pattern very small, complex features with many fewer steps relative to conventional methods,” he says. “The inks give the semiconductor industry a new path to more efficiently manufacture transistors with higher speeds.”
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