New finding could help circuits keep shrinking
In 1959 physicist Richard Feynman issued a famed address at a meeting of the American Physical Society, a talk entitled “There’s Plenty of Room at the Bottom.” It was an invitation to push the boundaries of the miniature, a nanotech call to arms that many physicists heeded to great effect. But more than 50 years since his challenge (pdf), researchers have begun to run up against a few hurdles that could slow the progression toward ever-tinier devices. Someday soon those hurdles could threaten Moore’s Law, which describes the semiconductor industry’s steady, decades-long progression toward smaller, faster, cheaper circuits.
One issue is that as wires shrink to just nanometers in diameter, their resistivity tends to grow, curbing their usefulness as current carriers. Now a team of researchers has shown that it is possible to fabricate low-resistivity nanowires at the smallest scales imaginable by stringing together individual atoms in silicon.
The group, from the University of New South Wales (U.N.S.W.) and the University of Melbourne in Australia, and from Purdue University in Indiana, constructed their wires from chains of phosphorus atoms. The wires, described in the January 6 issue of Science, were as small as four atoms (about 1.5 nanometers) wide and a single atom tall. Each wire was prepared by lithographically writing lines onto a silicon sample with microscopy techniques and then depositing phosphorus along that line. By packing the phosphorus atoms close together and encasing the nanowires in silicon, the researchers were able to scale down without sacrificing conductivity, at least at low temperatures.
“What people typically find is that below about 10 nanometers the resistivity increases exponentially in these [silicon] wires,” says Michelle Simmons, a U.N.S.W. physicist and a study co-author. But that appears not to be a problem with the new wires. “As we change the width of the wire, the resistivity remains the same,” she says.
Phosphorus is often introduced into silicon because each phosphorus atom donates an electron to the silicon crystal, which promotes electrical conduction or even can serve as bits in quantum computation schemes. But those conduction electrons can easily be pulled away from duty, especially in tiny wires where the wire’s exposed surface is large compared with its volume. By encasing the nanowires entirely in silicon, Simmons and her colleagues made the conduction electrons more immune to outside influence. “That moves the wires away from the surfaces and away from other interfaces,” Simmons says. “That allows the electron to stay conducting and not get caught up in other interfaces.”
Demonstrating electric transport in a wire so small “is quite an accomplishment,” says Volker Schmidt, a researcher at the Max Planck Institute of Microstructure Physics in Halle, Germany. “And being able to fabricate metallic wires of such dimensions, by this theoretically microelectronics-compatible approach, could be a potentially interesting route for silicon-based electronics.”
The wires, the researchers say, have the carrying capacity of copper, indicating that the technique might help microchips continue their steady shrinkage over time.
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