Chip Designers Of The Future Face Quantum Weirdness
November 7, 2012

Computer Chip Designers Face Hurdles For The Future Of Nanoelectronics

April Flowers for - Your Universe Online

How an electrical charge behaves when it is confined to metal wires only a few atom-widths in diameter is an essential piece of knowledge that designers need to understand to build the computer chips of the future.

Physicists from McGill University and researchers at General Motors R&D teamed up to show that when wires of two dissimilar metals meet, electrical current may be drastically reduced. This reduction in current is surprisingly sharp and reveals a significant challenge that could shape device design and material choices in the relatively new field of nanoelectronics.

Electronic circuit features are shrinking in size every year, thanks to the aggressive miniaturization prescribed by Moore's Law. Moore's Law postulates that the density of transistors on integrated circuits would double approximately every 18 months, making it possible to carry around computers in our pockets. This steady progress poses challenges though. The resistance to current no longer increases at a consistent rate as devices shrink. Instead, it jumps around, displaying counterintuitive effects of quantum mechanics.

"You could use the analogy of a water hose," McGill physics Professor Peter Grutter explains. "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 quantized ('jumpy') way."

The new study, published in Proceedings of the National Academy of Sciences, describes the "quantum weirdness" that the research team observed. Gold and tungsten are two metals currently used in combination in computer chips to connect different functional components of a device. The team investigated an ultra-small contact between the two metals.

Professor Grutter's lab handled the experimental side of the research, using advanced microscopy techniques to image a tungsten probe and a gold surface with atomic precision, and to mechanically bring them together in a precisely controlled manner. The resulting contact showed a much lower current than was expected. The GM R&D research team collaborated on mechanical modeling of the atomic structure of this contact.

McGill's physics department provided the state-of-the-art electrical modeling, confirming the lower current and showing dissimilarities in electronic structure between the two metals leads to a fourfold decrease of current flow. This is true even in a perfect interface. Another reason for the reduction in current was found to be crystal defects — displacements of the normally perfect arrangement of atoms — generated by the mechanical contact between the metals.

"The size of that drop is far greater than most experts would expect -— on the order of 10 times greater," notes Prof. Grutter. "The first step toward finding a solution is being aware of the problem," Grutter notes. "This is the first time that it has been demonstrated that this is a major problem for nanoelectronic systems."