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New surface chemistry may extend life of technology for making transistors
James
E. Kloeppel, Physical Sciences Editor
217-244-1073; kloeppel@illinois.edu
9/27/2004
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CHAMPAIGN,
Ill. — Researchers
at the University of Illinois at Urbana-Champaign have developed a technique
that uses surface chemistry to make tinier and more effective p-n junctions
in silicon-based semiconductors. The method could permit the semiconductor
industry to significantly extend the life of current ion-implantation
technology for making transistors, thereby avoiding the implementation
of difficult and costly alternatives.
To make faster silicon-based transistors, scientists much shrink the
active region in p-n junctions while increasing the concentration of
electrically active dopant. Currently about 25 nanometers thick, these
active regions must decrease to about 10 nanometers, or roughly 40 atoms
deep, for next-generation devices.
The conventional process, ion implantation, shoots dopant atoms into
a silicon wafer in much the same way that a shotgun sends pellets into
a target. To be useful, dopant atoms must lie close to the surface and
replace silicon atoms in the crystal structure. In the atomic-scale
chaos that accompanies implantation, however, many dopant atoms and
silicon atoms end up as interstitials – lodged awkwardly between
atoms in the crystal.
Ion implantation also creates defects that damage the crystal in a way
that degrades its electrical properties. Heating the wafer – a
process called annealing – heals some of the defects and allows
more dopant atoms to move into useful crystalline sites. But annealing
also has the nasty effect of further diffusing the dopant and deepening
the p-n junction.
“We developed a way of using surface chemistry to obtain shallower
active regions and enhanced dopant activation simultaneously,”
said Edmund Seebauer, a professor of chemical and biomolecular engineering
at Illinois. “By modifying the ability of the silicon surface
to absorb atoms from the substrate, our technique can control and correct
the defects induced during implantation.”
Inside the active region, atoms sitting on lattice sites have bonds
to four neighbors, which saturates the bonding capacity of the silicon
atoms. Atoms sitting on the surface have fewer neighbors, leading to
unused, or “dangling” bonds. Atoms of a gas such as hydrogen,
oxygen or nitrogen can saturate the dangling bonds.
“These dangling bonds can also react with interstitial atoms,
and remove them from the crystal,” Seebauer said. “The process
selectively pulls silicon interstitials to the surface, while leaving
active dopant atoms in place. The preferential removal of silicon interstitials
is exactly what is needed to both suppress dopant diffusion and increase
dopant activation.”
Seebauer and his colleagues – chemical and biomolecular engineering
professor Richard Braatz and graduate research assistants Kapil Dev
and Charlotte Kwok – use ammonia and other nitrogen-containing
gases to saturate some of the dangling bonds and control the ability
of the surface to remove interstitials.
“The amount of surface nitrogen compound formed, and therefore
the number of dangling bonds that become saturated, can be varied by
changing the type of gas and the degree of exposure,” Seebauer
said. “As an added benefit, nitrogen compounds are also quite
compatible with conventional chip manufacturing processes.”
Through computer simulations and experimental verification, the researchers
have shown that “defect engineering” by means of surface
chemistry can extend the life of current ion-implantation technology
and create smaller, faster electronic devices. Seebauer will present
the team’s latest findings at the 51st International Symposium
of the AVS Science and Technology Society, to be held Nov. 14-19 in
Anaheim, Calif.
Funding was provided by International SEMATECH and the National Science
Foundation. The researchers have applied for a patent.
