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Researchers find hurdles for future chips

Posted: 08 Nov 2012 ?? ?Print Version ?Bookmark and Share

Keywords:nanoelectronics? electrical charge? quantum weirdness? Proceedings of the National Academy of Sciences?

Calling attention to a future technology hurdle for semiconductor design, a team of physicists at McGill University have demonstrated that electrical current may be drastically reduced when wires from two dissimilar metals meet.

The surprisingly sharp reduction in current reveals a significant challenge that could shape material choices and device design in the emerging field of nanoelectronics, according to the researchers, who worked in collaboration with General Motors R&D.

As semiconductor feature sizes continue to shrink, designers of future chips will need to understand how an electrical charge behaves when it is confined to metal wires only a few atom-widths in diameter. As feature sizes dwindle to the level of atoms, the resistance to current no longer increases at a consistent rate as devices shrink, according to the McGill researchers. Instead the resistance "jumps around," displaying the counterintuitive effects of quantum mechanics, according to McGill physics professor Peter Grtter.

"You could use the analogy of a water hose," Grtter said. "If you keep the water pressure constant, less water comes out as you reduce the diameter of the hose. But if you were to shrink the hose to the size of a straw just two or three atoms in diameter, the outflow would no longer decline at a rate proportional to the hose cross-sectional area; it would vary in a quantised [jumpy] way."

Quantum weirdness
Grtter and his fellow McGill and GM researchers describe the "quantum weirdness" in a paper appearing in�Proceedings of the National Academy of Sciences.?The researchers investigated an ultra-small contact between gold and tungsten, two metals currently used in combination in semiconductors to connect different functional components of a device.

Grtter's lab used advanced microscopy techniques to image a tungsten probe and gold surface with atomic precision, and to bring them together mechanically in a precisely-controlled manner. The electrical current through the resulting contact was much lower than expected, they said. Mechanical modelling of the atomic structure of this contact was done in collaboration with Yue Qi, a research scientist with the General Motors R&D Centre in Warren, Mich.

Electrical modelling confirmed this result, showing that dissimilarities in electronic structure between the two metals leads to a fourfold decrease in current flow, even for a perfect interface, the researchers said. They also found that crystal defectsdisplacements of the normally perfect arrangement of atomsgenerated by bringing the two materials into mechanical contact was a further reason for the observed reduction of the current.

McGill student Till Hagedorn peers into a field ion microscope.

"The size of that drop is far greater than most experts would expecton the order of 10 times greater," Grtter said.

According to Grtter, the results point to a need for future research into ways to surmount this challenge, possibly through choice of materials or other processing techniques. "The first step towards finding a solution is being aware of the problem," Grtter said. "This is the first time that it has been demonstrated that this is a major problem" for nanoelectronic systems."

- Dylan McGrath
??EE Times





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