A Cool Way to Strip Hydrogen

A team of researchers from around the country has found a new process that could significantly alter the way that microchips and solar cells are made. Their work is published in this week’s issue of the journal Science.

“This is one of the dream projects of a scientist, in the sense that a significant step forward in some fundamental research may indeed influence the everyday life of the society,” said UT-ORNL joint professor of physics Zhenyu Zhang, a member of the research team. “If this discovery could eventually lead to a technology for fast and mass production of high purity silicon crystals at low growth temperatures, the whole semiconductor industry will benefit substantially, as will the solar energy industry.”

Chips made of silicon are in almost every piece of electronic equipment made today, from cell phones to computers to televisions and cars. In the process of creating the chips, layers of silicon are stacked to make the final product. After each layer of silicon is complete, though, hydrogen atoms are temporarily added to protect that layer from contamination. The hydrogen atoms are then removed so another layer of silicon can be added.

The team, made up of Zhang, Philip I. Cohen of the University of Minnesota and Leonard C. Feldman, Norman Tolk and Zhiheng Liu of Vanderbilt found a new way to remove the hydrogen atoms using lasers.

In the current process, silicon is heated to almost 900 degrees Fahrenheit, causing the bonds between the hydrogen and silicon to break. In the process, the silicon can become unstable and unusable, resulting in less final product.

To solve this problem, researchers tuned a laser to the specific frequency at which the two atoms vibrate. When the laser light and the bonds vibrated at the same frequency, the bonds absorbed the light, giving them a better chance to pair up and leave together from the surface. The advantage to this technique is that the hydrogen atoms can be removed at a low temperature and with great focus. Essentially, they are “cherry picked,” eliminating many of the problems associated with the current technique.

“We live in the silicon age,” said Tolk, a professor of physics at Vanderbilt. “The fact that we have figured out how to remove hydrogen with a laser raises the possibility that we will be able to grow silicon devices at very low temperatures, close to room temperature.”

In addition to applying this basic system to silicon surfaces covered only with hydrogen, they also tested it on surfaces covered with a mixture of hydrogen and its isotope deuterium. The researchers found that the technique can remove hydrogen atoms while leaving the deuterium atoms intact.

The potential for the new techniques expands far beyond just the semiconductor industry, though. This level of specific atomic control could be useful in fields like nanotechnology, where the ability to control the growth of structures on an incredibly small level could allow scientists to move past a major stumbling block in the research.

Vanderbilt, the University of Minnesota and ORNL are filing a joint patent on the process and its potential applications. The experimental work reported in the Science article was conducted at Vanderbilt’s W.M. Keck free-electron laser center, and was motivated by a theoretical prediction, reported in a 2004 issue of Applied Physics Letters by Biao Wu of ORNL along with Cohen, Feldman and Zhang.

Zhang joined UT in 1997, two years after coming to East Tennessee as a research scientist at ORNL. He has been a UT-ORNL joint professor since 2003. Zhang earned his Ph.D. in physics at Rutgers University in 1989 and is a fellow of the American Physical Society.

UT is the managing partner of ORNL through the UT-Battelle partnership.

From NSF