Dose-rate effects on the formation of ultra-shallow junctions with low-energy B+ and BF2+ ion implants

B-11(+) and (BF2+)-B-49 implants on a Varian VIISion-80 PLUS Ion Implanter from 2.0 to 8.9 keV at a dose of 1E15/cm(2), and at various controlled and measured (in situ) peak beam-current densities, ranging from 3 to 600 mu A/cm(2), were investigated to study the effects of dose rate on the formation of ultra-shallow junctions. The implants and annealing conditions were chosen to produce junction depths, as measured by secondary ion mass spectrometry (SIMS), of 40 to 150 nm. In addition, a comprehensive study of B vs. BF2 at a boron effective energy of 2.0 keV (i.e. B at 2.0 keV and BF2 at 8.9 keV) was undertaken. The results show that for the implant conditions investigated the dose rate does not have a significant effect (if any) on the junction depth and that there is a distinct advantage to BF2 implants in forming shallower junctions. This advantage is not dose-rate related, but is related to the presence of fluorine. This paper also addresses the effects of preamorphization with Ge on dopant activation and on transient enhanced diffusion, and annealing techniques to optimize sheet resistance while minimizing junction depths. The background concentration of O-2 during anneal was found to have a dramatic impact on the annealed junction (from oxidation-enhanced diffusion). Reducing the O-2 concentration to tract amounts, produced the shallowest junctions observed. By combining those techniques which reduce boron diffusion, junctions that were only 39 nm deep, having a sheet resistance of 361 Omega/sq., were fabricated with 5 keV BF2. (C) 1997 Elsevier Science S.A.