Researchers model ion bombardment for better nanofabrication

May 22, 2012 — Brown University is using supercomputer simulations to better understand the ion bombardment of metal surfaces used in manufacturing nanoelectronics.

The research shows what takes place during ion bombardment in trillionths of a second. It could lead to better prediction and more uses of ion bombardment to make nanoscale structures.

Figure. Three new mechanisms at the nanoscale. A computer-model image of an island of metal atoms formed after bombardment by noble gas ions. Atoms disturbed by the bombardment cluster together under the surface and then glide back up in a matter of 2.1ps. SOURCE: Kim Lab/Brown University.

Ion bombardment is used in semiconductor and optoelectronics manufacturing, but has been limited by the lack of understanding of the underlying physics. Brown University engineers modeled noble gas ion bombardments to provide insights into how ion bombardment works, with the hopes of better predicting what surface patterns and stresses would result.

The research applies to “FCC” metals — copper, silver, gold, nickel, aluminum — crystals made up of cubic arrangements of atoms with one at each corner and one in each cube-face center.

This research builds on previous attempts to model ion bombardment on computers, by modeling more than one bombardment event or isolated point defects in the metal substrate. This work investigated collective behavior of defects during ion bombardments in terms of ion-substrate combinations, said Kyung-Suk Kim, professor of engineering at Brown.

The new model revealed how ion bombardments can set three main mechanisms — “dual layer formation,” “subway-glide mode growth,” and “adatom island eruption” — into motion in a matter of picoseconds. The mechanisms are a consequence of how the incoming ions melt the metal and then how it re-solidifies with the ions occasionally trapped inside.

When ions hit the metal surface, they penetrate it, knocking away nearby atoms in a process that is akin, at the atomic level, to melting. But rather than merely rolling away, the atoms resolidify in a different order.

Some atoms have been shifted out of place. There are some vacancies in the crystal nearer to the surface, and the atoms there pull together across the empty space, that creates a layer with more tension. Beneath that is a layer with more atoms that have been knocked into it. That crowding of atoms creates compression. Hence there are now two layers with different levels of compression and tension. This “dual layer formation” is the precursor to the “subway-glide mode growth” and “adatom island eruption”.

Materials that have been bombarded with ions sometimes produce a pattern of material that seems to have popped up out of the original surface. Scientists theorized that displaced atoms would individually bob back up to the surface, but the team’s models shows that these molecular islands are formed by whole clusters of displaced atoms that bond together and appear to glide back up to the surface, like passengers converging to emerge from a subway car.

Kyung-Suk Kim noted that “predictive design capability for controlling the surface patterns and stresses in nanotechnology products” could lead to flexible electronics, biocompatible surface formation on medical devices, and other new technologies.

“As a next step, I will develop prediction models for nanopattern evolution during ion bombardment which can guide the nanomanufacturing processes,” said Sang-Pil Kim, postdoctoral scholar. “This research will also be expanded to other applications such as soft- or hard-materials under extreme conditions.”

The research will be published May 23 in the Proceedings of the Royal Society A. Sang-Pil Kim is lead author, with Kyung-Suk Kim, Huck Beng Chew, Eric Chason, and Vivek Shenoy.

The research was funded by the Korea Institute of Science and Technology, the U.S. National Science Foundation, and the U.S. Department of Energy. The work used the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by National Science Foundation grant number OCI-1053575.

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