More light, less heat

June 7, 2006
Laser light rather than heat can strip hydrogen atoms from silicon surfaces, sayresearchers at the University of Minnesota, Vanderbilt, the University ofTennessee, and Oak Ridge National Laboratory.

The breakthrough could lowercosts and boost quality of computer chips and solar cells.

Microelectronic devices are built from multiple layers of silicon. To keepsilicon surfaces from oxidizing, semiconductor manufacturers routinely exposethem to hydrogen atoms that attach to all the available silicon bonds. However,this "passivation" process means that the hydrogen atoms must be removedbefore adding new layers of silicon. "Desorbing" the hydrogen requires hightemperatures that introduce thermal defects and reduce chip yields. The abilityto remove hydrogen with a laser could make possible the growing of silicondevices at close to room temperature, say researchers.

FETs that run at speeds about 40% faster than ordinary transistors are onetarget of the technology. Lowering process temperature by 100°C shoulddramatically improve yields.

Researchers used a highly tunable, free-electron laser for the work. Mostlasers produce light only in a few distinct frequencies. The FEL operates in theinfrared portion of the spectrum, which is particularly valuable for probing thestructure and behavior of materials.

Specifically, laser light was tuned to the frequency at which the hydrogensiliconbonds vibrate and polarize so the photon's electrical field is pointed in thesame direction as the silicon-hydrogen bonds. The technique also works onsurfaces covered with a mixture of hydrogen and its isotope deuterium. It canremove hydrogen atoms while leaving the deuterium atoms intact.

This degree of selectivity could provide a way to control the growth ofnanoscale structures with an unprecedented degree of precision. By selectivelyremoving the hydrogen atoms from the ends of nanowires, it should be possibleto control and direct their growth, which currently is a random process.

Driving chemical reactions along nonthermal pathways is another potentialapplication. When a molecule heats up, the weakest bond breaks first. Attemptsto tune lasers to break stronger bonds so far have been thwarted by the rapiditywith which molecules convert light into thermal energy.

Funding for the project comes from the DoE and Darpa.

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