Laser to break specific molecular bonds

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.

"We live in the silicon age," observes Norman Tolk, 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 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. 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 oxidising, semiconductor manufacturers routinely expose them to hydrogen atoms that attach to all the available silicon bonds.

However, this process known as "passivation' 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% 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 1000C which should dramatically improve yields.

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."

The research was carried out at Vanderbilt's W. M. Keck Free-electron Laser Center.

The University of Minnesota is filing a patent on the application of the laser desorption method to silicon processing.