In the Game

New Molecule Could Help Solve Microchip Puzzle

By Doug Peterson

December 2005

New molecules offer new possibilities in the quest to build transistors as small as only a few hundred atoms.

In a unique collaboration, Greg Girolami, University of Illinois professor of chemistry, synthesized a series of new volatile molecules, each of which contains a transition metal atom surrounded with boron-hydrogen groups. Then John Abelson, CSL researcher and professor of materials science and engineering, used the molecules to deposit thin films of transition metal diborides.

These thin film materials are “metallic ceramics,” which conduct electricity as effectively as metals and have the extremely high melting temperatures of ceramics, Abelson said. They can be used to create layer upon layer of fine connections between the millions of transistors on a microchip.

“When we think about computer chips, we often focus on the transistors,” said Abelson. “But the part most people don’t think about is the complex wiring between the transistors.”

As the size of transistors continues to shrink to nanoscale dimensions, everything becomes more complicated. For instance, Abelson said, seven different levels of wiring are built on top of a typical silicon chip, each consisting of an insulating layer and a network of metal lines. To provide electrical connection, thin film metals must be deposited evenly within tiny “via” holes that connect these levels. But with shrinking dimensions, it is getting exceedingly difficult to deposit the thin film metals evenly.

That’s where the chemistry comes in.

One process used to deposit thin film metals is called chemical vapor deposition. As Abelson explained, “The first word in the process is ‘chemical.’ Some years ago I realized that none of us doing research on thin film deposition were chemists -- and that working with an excellent chemist was guaranteed to open up new possibilities.”

So Abelson struck up a working relationship with chemistry professor Girolami. The result was a series of new molecules that are uniquely suited to deposit thin film materials evenly in the via holes of future-generation computer chips.

“We have the only gram of the molecule that has ever existed,” he said.

Abelson creates the new thin film material by flowing molecules, in gas form, over a substrate that is heated to only 200°C (392°F). The low temperature is essential, he said, because higher temperatures can cause the electrically active “dopants” in the silicon transistors to move around -- which is not desirable.

The new transition metal diboride thin film materials potentially could eliminate one or two steps in the process of creating microchips. This could translate into big savings for microchip manufacturers, which sink about $30,000 of processing into each silicon wafer (roughly the size of a Frisbee).

What’s more, Abelson’s new thin film material is resistant to oxidation and is mechanically durable, so it may also be used as a protective hard coating.

Reliability is essential in the ultra-complex process of manufacturing microchips. There have to be no failed parts in the circuitry of millions of transistors on a chip -- which is why “the modern integrated circuit factory costs upwards of a billion dollars,” Abelson said. “The whole thing runs at an extraordinarily high level of perfection. It’s amazing it works.”

Because of the expense involved, industry is very careful before delving into new ways of doing things. But companies are convinced that the shrinking dimensions of microchips will eventually force them to use some variant of chemical vapor deposition to make thin films such as Abelson’s.

“The question is: Which new process will they choose?” Abelson said. “We’re a relative newcomer. But in the last two years, we’ve gotten some exciting results. Only time will tell which process meets all of the metrics. But that’s part of the fun. We’re in the game and gaining speed.”

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