Molecular implants

The irradiation of materials with molecular ions shows different features with respect to traditional monatomic ion irradiation, mainly related to the multiple collisions caused by the cluster impact on the surface region of the target. This results in the motion of many surface atoms and the enhancement of chemical reactions. Molecular implants are used for cleaning and nanopatterning of solid surfaces, as well as for the creation of thin films. In the particular case of the microelectronics industry, C clusters are implanted in silicon to create tensile layers, and the use of B molecules for doping has been proposed to overcome some of the limitations of monatomic B implantation, related to production-throughput and ion-beam high-energy contamination.

Using classical molecular dynamics techniques, we carried out a comparative study of monoatomic B and octadecaborane (B₁₈) cluster implantations in order to determine the advantages and drawbacks of each approach regarding the fabrication of shallow junctions. We found that at typical implantation doses and energies, obtained implantation profiles are very similar. However, damage generation is totally different. While monoatomic B is not able to amorphize the silicon substrate, octadecaborane implantation is self-amorphizing. This is a great advantage, since decreases B channeling. Recrystallization of the amorphous layer in a subsequent thermal treatment is able to fully activate dopants and remove damage from the layer up to the substrate surface. Consequently, cluster implantation produces much less residual damage than monoatomic B, which help to reduce the transient enhanced diffusion of dopants.

Typical damage created by a monatomic B and B₁₈ cluster implantations. B energy is the same in both cases, 500 eV. Red lines show the B trajectories along the simulations (L. A. Marqués et al., Phys. Rev. B 74, 201201 (2006) - Characterization of octadecaborane implantation into Si using molecular dynamics).