Scientists apply “laser” focus to strip hydrogen from silicon

May 22, 2006 – Researchers from the U. of Minnesota, Vanderbilt U., and the U. of Tennessee and Oak Ridge National Laboratory have developed a method to strip hydrogen atoms from silicon surfaces, a means that could enable production of silicon devices at nearly room temperature.

Silicon surfaces are exposed to hydrogen atoms in a “passivation” process to prevent oxidation during the semiconductor manufacturing process. The hydrogen atoms attach to all available silicon bonds, and must be removed before another silicon layer is applied. Desorbing the hydrogen thermally requires high temperatures, which can create thermal defects in the chips.

In previous work using lasers, targeted molecules quickly absorbed and converted the light energy into heat, which indiscriminately broke the weakest bonds first leaving targeted ones intact. In the new process, researchers used a free-electron laser operating in the infrared portion of the spectrum to tune the light to the frequency at which the hydrogen-silicon bonds vibrated and polarized, so that the photon’s electrical field was pointed in the same direction as the silicon-hydrogen bonds.

“By selectively removing the hydrogen atoms from the ends of nanowires — structures typically 5-10 nm wide — we should be able to control and direct their development, which currently is a random process,” stated U. Minn professor Philip Cohen. He indicated the technique could manufacture field-effect transistors (FETs) that operate about 40% faster than ordinary transistors, while reducing processing temperatures by 100°C. It could also be used to extend a silicon crystal to make airtight seals around minuscule sensors.

“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,” added Vanderbilt prof. Norman Tolk.

Vanderbilt, the U. of Minnesota, and Oak Ridge National Labs are filing a joint patent on the process and its potential applications. Their work was described in the May 19 issue of the journal Science.


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