Portland, Ore. - Downsizing to the nanoscale theoretically endows materials with greater strength, enabling copper interconnects, for example, to remain reliable even when line widths shrink to the nanoscale. But critics predict that high temperatures during use might cause the nanometer-sized grains to merge back into micron-sized grains, thereby making the material more brittle and prone to failure.
Now a University of Arkansas professor contends that his simulations foretell a day when nanoscale copper will be doped, just as silicon semiconductors are today, thereby stabilizing the metal and preventing it from becoming brittle at high temperatures.
"Others have predicted that nanoscale materials will lose their good properties at high temperatures, but our study indicates that a dopant with a much larger atom can counter the effect by migrating to grain boundaries and lowering the energy there," said professor Panneer Selvam at the University of Arkansas (Fayetteville), who performed the work there with Ashok Saxena, dean of the College of Engineering, and graduate students Paul Millett and Shubhra Bansal.
Today the size of the copper grains in an interconnect can be measured in microns, but in the future crystalline copper films could perform the interconnect function with grain sizes measured in nanometers.
Using the same molecular-dynamics simulator as the critics who predict high-temp nanoscale meltdown, Selvam's team of researchers created "polycopper," a nanoscale copper bicrystal using a triangular lattice with two orientation angles that yields a 20- to 30-nanometer grain size.
By the laws of molecular dynamics, polycopper with a 20-nm grain size should be 10 times stronger than normal, micron-grain-size copper. By the same laws of molecular dynamics, however, the polycopper should also revert to normal copper at high temperatures, thereby losing its extra strength. Even at temperatures as low as 125 degrees C (257 degrees F), the grain size of undoped polycopper increased from 20 to 50 nm.
But by doping the polycopper with antimony the researchers saw that it did not alloy (mix) with the polycopper but instead migrated as individual atoms of antimony to the boundaries between copper grains, thereby relaxing the energy there. Because the antimony atoms are more than twice as large, they act as a buffer zone that lets the material expand and contract slightly between grains as temperatures change, without disrupting the insides of the grains or their place in the polycrystalline atomic lattice.
"We began it as a two-dimensional problem, which was appropriate for thin films, but now we want to expand it to three dimensions to deepen our understanding. Also, now we are only simulating about 1,800 atoms, so next we want to work on much larger systems, with 10,000 atoms or more," said Selvam.
Beside expanding the model to three dimensions, the researchers plan on replacing their current mathematical model, using the "Lennard-Jones potential," to a full-blown "many-body potential" that can more accurately simulate metallic bonding, Selvam said. His group also hopes to characterize a fully nanocrystalline sample over varying temps to map the exact mechanics involved as dopant atoms near the grain boundary help to maintain a nanoscale average grain size.