May 19, 2006, Minneapolis/St. Paul, MN--A team of researchers have used laser light (instead of heat) to break specific molecular bonds--such as stripping hydrogen atoms from silicon surfaces. "We live in the silicon age," said Tolk, who is a physics professor 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."
The new technique was developed by Philip Cohen, a professor of electrical and computer engineering at the University of Minnesota, working with Vanderbilt University researchers Leonard Feldman, Norman Tolk and Zhiheng Liu along with Zhenyu Zhang from Oak Ridge National Laboratory. It is described in the May 19 issue of the journal Science.
Microelectronic devices are built from multiple layers of silicon. In order to keep silicon surfaces from oxidizing, semiconductor manufacturers routinely "passivate" them by exposing them to hydrogen atoms that attach to all the available silicon bonds. However, this means that the hydrogen atoms must be removed before new layers of silicon can be added. "Desorbing" the hydrogen is usually done by heating to high temperatures (800 C), which can create thermal defects in the chips and so reduce chip yields.
"One application that we intend to examine is the use of this technique to manufacture field effect transistors (FETs) that operate at speeds about 40 percent faster than ordinary transistors," said Cohen. According to Cohen, it should be possible to reduce the processing temperature of manufacturing FETs by 100 degrees Celsius, which should dramatically improve yields.
The research was carried out at Vanderbilt's W.M. Keck Free-electron Laser Center. Because the silicon/hydrogen system has been intensively studied, the researchers knew the strength of the bond between the silicon and hydrogen atoms. Like tiny springs, the bonds tend to vibrate at certain frequencies and are most likely to absorb light photons that vibrate at these frequencies. As a result, light tuned to these "resonant" frequencies can force the bond to break. When the researchers scanned the laser through the frequencies that they had calculated would resonate with the silicon-hydrogen bond, they found that the rate of hydrogen desorption peaked at an incident wavelength of 4.8 microns.
The process could be used to control the growth of nanoscale structures with an unprecedented degree of precision, and it is this potential that most excites Cohen. "By selectively removing the hydrogen atoms from the ends of nanowires, we should be able to control and direct their growth, which currently is a random process," he said.
