A team of researchers has achieved a long-sought scientific goal: using laser light to break specific molecular bonds. The process uses laser light, instead of heat, to strip hydrogen atoms from silicon surfaces. This is a key step in the manufacture of computer chips and solar cells, so the achievement could reduce the cost and improve the quality of a wide variety of semiconductor devices.
Laser photons (wavy red lines) can clear hydrogen atoms (grey and purple) from the surface of silicon (yellow) at much lower temperatures than conventional techniques. (Credit: Brian Muller, Vanderbilt University)
The technique was developed by Philip I. Cohen at the University of Minnesota, working with Vanderbilt researchers Leonard C. Feldman, Norman Tolk and Zhiheng Liu along with Zhenyu Zhang from Oak Ridge National Laboratory and the University of Tennessee. It is described in the May 19 issue of the journal Science.
"We live in the silicon age," observes 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."
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 thermally requires high temperatures and adds substantially to difficulty of process control because these temperatures 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," says Cohen. According to the professor of electrical and computer engineering, it should be possible to reduce the processing temperature of manufacturing FETs by 100 degrees Celsius which should dramatically improve yields.
Vanderbilt, the University of Minnesota and Oak Ridge National Laboratory are jointly filing a patent on the process and its potential applications.
In addition to a wide range of potential applications, the discovery has important scientific implications. Since the invention of the infrared laser, chemists have been trying to use it to drive chemical reactions along non-thermal pathways. But, as Yale chemist John C. Tully remarks in an accompanying commentary in Science, "molecules have not cooperated." When a molecule is heated up, the weakest bond breaks first. Attempts to tune lasers to break stronger bonds have been repeatedly thwarted by the rapidity with which molecules convert the light energy into thermal energy. Describing the new findings as a "striking contrast" to previous studies, Tully observes that the researchers have "successfully accomplished a long-standing goal."
At this point, the researchers have a number of speculations on the reasons why their technique succeeds where so many others have failed. The main clue is the totally unexpected observation that the hydrogen atoms appear to detach from the surface in pairs, as hydrogen molecules, rather than as individual atoms. Additional research will be needed to work out the atomic mechanism involved.
The research was carried out at Vanderbilt's W. M. Keck Free-Electron Laser Center. The free-electron laser is a special kind of laser which has the advantage that its beam can be tuned through a wide range of frequencies in much the same way that you can dial up different frequencies on a radio. Most lasers only produce light in a few distinct frequencies. The Vanderbilt FEL operates in the infrared portion of the spectrum, which is particularly valuable for probing the structure and behavior of materials.
The silicon/hydrogen system has been intensively studied. So the researchers knew the strength of the bond between the silicon and hydrogen atoms. The bonds between atoms act something like an atomic spring. Like tiny springs, they tend to vibrate at certain frequencies and are most likely to absorb light photons that vibrate at the same frequencies. As a result, light tuned to one of these "resonant" frequencies can force the corresponding 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, which corresponds directly to the known frequency of the Si-H spring.
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 deuterium. Deuterium is an isotope of hydrogen: Instead of the single proton that hydrogen has as a nucleus, deuterium has a proton and a neutron: It has the same chemical characteristics as hydrogen but it weighs about twice as much. This weight difference means that the silicon-deuterium bond vibrates more slowly than the silicon-hydrogen bond and the wavelength of the photons that excite it is very different.
Once they got the set up right, the researchers found that it worked far better than they ever expected. They discovered that the laser desorption process:
* Works efficiently even at room temperature. Prior theoretical work by Zhang, Feldman, Cohen and Biao Wu, then a post doctoral fellow at Oak Ridge National Laboratory, predicted that a substantial fraction of the hydrogen atoms could be excited by the laser, but temperatures well above room temperature would be needed to desorb the hydrogen efficiently.
* Generates surprisingly little heat. In the infrared wavelengths used by the researchers, silicon is basically transparent. So photons that don't contribute to the desorption process are either reflected by the surface or pass completely through it. In the experiments with hydrogen-deuterium mixtures, less than 5 percent of the desorbed material was in the form of hydrogen-deuterium or deuterium-deuterium pairs. This indicates that very little of the laser energy involved in desorbing the hydrogen is converted to heat.
* Exhibits a high degree of selectivity. With the hydrogen/deuterium mixture, the researchers demonstrated that they can remove large numbers of hydrogen atoms without detaching many of the deuterium atoms. This result has given the researchers confidence that they can selectively remove hydrogen atoms from certain surface locations, but not others. For example, the strength of the bonds between the hydrogen atoms and the silicon atoms located along stair-step edges (common surface features) are stronger than the hydrogen-silicon bonds on flat surfaces. The researchers believe they can tune the laser with enough precision to detach only the stair-step hydrogen atoms and plan to do so in a future experiment.
Selectivity of this kind could provide a way to control the growth of nanoscale structures with an unprecedented degree of precision and it is this potential that most excites Cohen, who notes, "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."
Feldman, the Stevenson Professor of Physics at Vanderbilt, maintains that the process represents a significant advance in the ability to modify the surfaces of materials at the atomic level. "We have a new way to selectively interrogate and modify surfaces. If you stop to think about it, surfaces are where the action is. It is where the rubber meets the road! So, not only will this new technique allow us to create innovative new devices, it will also provide us with invaluable new knowledge about vital surface processes. In fact, some of the most advanced nanotechnology devices that have been envisioned, like quantum computers, require the level of control that atom-specific processes of this sort make possible."
Zhiheng Liu is a post-doctoral fellow at Vanderbilt and Zhenyu Zhang is a condensed matter theorist at Oak Ridge National Laboratory and the University of Tennessee.
The project was supported by grants from the Department of Energy, the Defense Advanced Research Projects Agency and the National Science Foundation.