To modify a metal surface at the scale of atoms and
molecules—for instance to refine the wiring in computer chips or the reflective
silver in optical components—manufacturers shower it with ions. While the
process may seem high-tech and precise, the technique has been limited by the
lack of understanding of the underlying physics. In a new study, Brown University
engineers modeled noble gas ion bombardments with unprecedented richness,
providing long-sought insights into how it works.
“Surface patterns and stresses caused by ion beam
bombardments have been extensively studied experimentally but could not be
predicted accurately so far,” said Kyung-Suk Kim, professor of engineering at
Brown and coauthor of the study published in the Proceedings of the Royal Society A. “The new
discovery is expected to provide predictive design capability for controlling
the surface patterns and stresses in nanotechnology products.”
The improved understanding could open the door to new
technologies, Kim said, such as new approaches to make flexible electronics,
biocompatible surfaces for medical devices, and more damage-tolerant and
radiation-resistant surfaces. The research applies to so-called “FCC” metals
such as copper, silver, gold, nickel, and aluminum. Those metals are crystals
made up of cubic arrangements of atoms with one at each corner and one in each
Scientists have been trying to explain the complicated
process for decades, and more recently they have begun to try modeling it on
computers. Kim said the analysis of the Brown team, including lead author and
postdoctoral scholar Sang-Pil Kim, was more sophisticated than previous
attempts that focused on a single bombardment event and only isolated point
defects within the metal substrate.
“In this work, for the first time, we investigate
collective behavior of those defects during ion bombardments in terms of
ion-substrate combinations,” Kyung-Suk Kim said.
The new model revealed how ion bombardments can set three
main mechanisms into motion in a matter of trillionths of a second. The
researchers dubbed the mechanisms “dual layer formation,” “subway-glide mode
growth,” and “adatom island eruption.” They are a consequence of how the
incoming ions melt the metal and then how it resolidifies with the ions
occasionally trapped inside.
When ions hit the metal surface, they penetrate it,
knocking away nearby atoms like billiard balls in a process that is akin, at
the atomic level, to melting. But rather than merely rolling away, the atoms
are more like magnetic billiard balls in that they come back together, or
resolidify, albeit 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”.
A hallmark of materials that have been bombarded with
ions is that they sometimes produce a pattern of material that seems to have
popped up out of the original surface. Previously, Kyung-Suk Kim said,
scientists thought displaced atoms would individually just bob back up to the
surface like fish killed in an underwater explosion. But what the team’s models
show is that these molecular islands are formed by whole clusters of displaced
atoms that bond together and appear to glide back up to the surface.
“The process is analogous to people getting on a subway
train at suburban stations, and they all come out together to the surface once
the train arrives at a downtown station during the morning rush hour,”
Kyung-Suk Kim said.
The mechanisms, while offering a new explanation for the
effects of ion bombardment, are just the beginning of this research.
“As a next step, I will develop
prediction models for nanopattern evolution during ion bombardment which can
guide the nanomanufacturing processes,” Sang-Pil Kim said. “This research will
also be expanded to other applications such as soft- or hard-materials under