Surface chemistry may extend life of transistors

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 extend the life of current ion-implantation technology for making transistors, avoiding difficult and costly alternatives.

To make faster silicon-based transistors, scientists must shrink the active region in p-n junctions while increasing the concentration of electrically active dopant.

Currently about 25 nanometres thick, these active regions must decrease to about 10 nanometres, 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.

By modifying the ability of the silicon surface to absorb atoms from the substrate, the technique can control and correct the defects induced during implantation.

Inside the active region, atoms sitting on lattice sites have bonds to four neighbours, which saturates the bonding capacity of the silicon atoms. Atoms sitting on the surface have fewer neighbours, leading to unused, or 'dangling' bonds.

Atoms of a gas such as hydrogen, oxygen or nitrogen can saturate the dangling bonds.

The dangling bonds can also react with interstitial atoms and remove them from the crystal. 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.

Ammonia and other nitrogen-containing gases are used 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 the number of dangling bonds that become saturated, can be varied by changing the type of gas and the degree of exposure. As an added benefit, nitrogen compounds are also quite compatible with conventional chip manufacturing processes.