Future generations of Si electronic devices will need very shallow p-n junctions, in the tens of nanometer range. Implantation of B to form p-type junctions of such low depth requires very low energies, below 1 keV, where the ion beam formation and transport are hindered by space-charge effects. Shallow implantation also can be achieved using higher energy beams of ionized large molecules, such as decaborane (B 10 H 14 ), since the atoms are implanted with only a fraction of the beam energy. Measurements of electron impact ionization and breakup of decaborane in the electron energy range, 25-260 eV, and temperatures up to 350ЊC are reported here. Ions containing 10 B atoms were found to be the dominant component in all measured mass spectra. In another set of experiments, the beams of the B 10 H x ϩ cluster ions were generated in an electron impact ionization source, mass analyzed, transported through a 2.5 m long ion beam line, and implanted into Si. No significant breakup of the ions and no neutral beam component were found. Beams of ions with ten B atoms were formed more easily and are more robust than initially thought. The results confirm the potential of decaborane cluster ions for low energy implantation of boron.

Ion beams of decaborane (B1,,HI4) are used to form ultra shallow p-type junctions in Si. Because the ion energy is partitioned between the atoms of the molecule, B atoms are implanted with only approximately one tenth of the energy of the beam. Thus severe problems created by the space charge of ultra low energy (ULE) ion beams are minimized. Moreover, standard ion implanters equipped with a decaborane ion source may be capable of ultra shallow (tens of nm) implantation of boron. Ionization and ion beam properties of decaborane were studied in the energy range of 2 -10 kV. Under proper conditions in the ion source, most of the extracted ions consist of 10 B atoms (B1&+) and they can be transported through the implanter without significant break-up or neutralization. Boron depth profiles measured by SIMS in Si wafers implanted with BloH,' and B' ions of equivalent energy are the same but it appears that the retained dose achieved with the molecular ions is higher than with the monomer ions for the same B fluence. The effect may be due to a different Si sputtering yield per impinging B atom with the two types of ions. Si wafers with test MOS devices were implanted with decaborane ions and ULE BF2' ions of equivalent energy. Measured device characteristics are very similar. The results confirm the potential of decaborane ion beams as an alternative technology for manufacturing of ultra-shallow p-type junctions in Si.

Decaborane cluster ions (B10Hx+) may play an important role in the manufacturing of future semiconductor devices, as they facilitate a very shallow implantation of B with a relatively high beam energy, due to its partition among the constituent atoms. While the formation of B-doped shallow junctions in Si has been demonstrated, little is known about other effects of these complex ions on a solid. We have measured the sputtering yield of Si with decaborane cluster ions at 12 keV and demonstrated that their impacts smooth rather than roughen the surface, similarly to much larger Ar cluster ions. The results have implications for the understanding of low-energy atomic impacts in terms of collective motion of many surface atoms and of the behavior of solids far from equilibrium.

Formation of p-type shallow junctions for future generations of Si devices will require ion implantation of B at very low energies (< 1 keV). An alternative to implantation of monomer ions at very low energy is implantation of large molecular ions at a higher energy. In an ion beam of decaborane (B10H14) each of the B atoms carries only 9% of the ion kinetic energy. We have examined ionization properties of decaborane and built an experimental ion source and an implantation apparatus with magnetic mass analysis. Analyzed decaborane ion beams with energies from 2 to 10 keV and beam currents of several microamperes were obtained. Si samples were implanted with decaborane ions and the implanted dose measured by current integration was compared with B content obtained by nuclear reaction analysis. Experiments with electrostatic beam deflection show that the large ions survive the transport in the implanter environment and that neutralization is negligible. During implantation, the retained B dose is reduced in comparison with the nominal implanted dose due to sputtering. Dose loss is greater at 200 eV compared to 500 eV. The properties of decaborane ion beams and the prospects of using them for shallow implantation of B into Si are discussed.

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 ...