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May 18, 2006

more precise control on the way to full MNT: Researchers achieve long-sought goal of using lasers to break specific molecular bonds

Stripping hydrogen from silicon surfaces with light instead of heat promises to improve the quality of computer chips and solar cells 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.

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 oxidizing, 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 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.

This degree of selectivity 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."

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