Transient Enhanced Diffusion of B in Si Implanted with Decaborane Cluster Ions

Abstract

Cluster ions, obtained by ionization of decaborane (B 10 H 14 ) vapor are considered for implantation of B into Si to form very shallow junctions required for the next generations of metal oxide semiconductor devices. While shallow B implantation can thus be achieved with relatively large implantation energies, the final junction depth is also defined by diffusion during annealing, particularly by transient enhanced diffusion ͑TED͒. TED of B in Si implanted with mass analyzed B 10 H x ϩ cluster ions at energies of 2, 5, and 12 keV is compared with TED from 1.2 keV B ϩ ions. No difference was found between TED of B implanted in Si with the cluster and the monomer ions of the equivalent energy and dose.Ion implantation, which for over 30 years has been the main technique of doping semiconductors, faces major challenges at the low ion energies required for the formation of very shallow junctions of future generations of silicon devices. The International Technology Roadmap for Semiconductors projects the need for 20 to 33 nm junctions in the 0.1 m devices projected for the year 2005. 1 The problem is especially severe for the light ion B ϩ that needs to be implanted at energies Ͻ1 keV for such shallow depths. Extraction from an ion source and transport of such low energy ions are hindered by the Coulomb forces ͑beam space-charge͒ severely limiting throughput for commercial semiconductor implantation. An attractive approach that overcomes this problem is based on energetic beams of cluster ions, which produce implantation effects equivalent to those of monomer ions at a lower energy, due to partitioning of the cluster kinetic energy among its constituent atoms. Thus, each B atom in a B 10 ϩ cluster ion carries only one-tenth of the beam energy. Moreover, the charge for a given atom fluence decreases by the same factor, reducing the beam space-charge as well as wafer charging.The final depth profiles of B in Si are defined as much by the implantation as by diffusion during dopant activation annealing ͑see Fig. 1͒. In the case of shallow B-doped junctions, the critical phenomenon is transient-enhanced diffusion ͑TED͒, which may exceed equilibrium diffusion by orders of magnitude. 2 In assessing the feasibility of the formation of ultrashallow junctions with cluster ions, it is important to show that TED in this case is at least not larger than in the case of B ϩ ions.For boron, a cluster of ten atoms exists naturally in the form of a molecule of decaborane, B 10 H 14 . Decaborane implantation into Si, first reported in 1996, was used for fabrication of an experimental p-type metal oxide semiconductor ͑MOS͒ transistor by Fujitsu in collaboration with Kyoto University, where the implantation was performed. 3 The ion beam, however, was not mass analyzed, and the exact nature of the implanted species could not be confirmed. It was suggested that implantation of B using decaborane ions may reduce TED. Subsequent measurements of TED on Si samples, implanted with 5 keV decaborane ions, showed that TED appears to be the same ...