Issues on boron electrical activation in silicon: Experiments on boron clusters and shallow junctions formation

Abstract

A comprehensive understanding of dopant activation mechanisms in crystalline Si is required in order to form shallow junctions. In this paper, we will review several experimental assessments on boron clustering and novel methods to form shallow junctions. Boron marker-layer structures have been used to investigate the fundamental aspects of formation and ripening boron-interstitial clusters (BICs) and their influence on the associated transient enhanced diffusion (TED). The samples were damaged by Si implants at different doses in the sub-amorphizing range and annealed at high temperatures. We found that BICs act as a sink for interstitials at supersaturations values S( t)>10 4. This implies that silicon self-interstitial defects are the primary source of interstitials driving TED, and that BICs act as a secondary “buffer” for the interstitial supersaturation. These clusters are less sensitive to the ripening process than pure interstitial clusters do, so that their size remain below 2 nm regardless of the annealing time. Moreover, we found that BICs formed in the early stage of the annealing reduce considerably the active amount of B. An extensive characterization of the electrical activation of ultra-low energy implanted boron in silicon is also reported. The spreading resistance profiling (SRP) technique has been used, in a suitable configuration, for measuring doped layers shallower than 100 nm, in order to extract the carrier concentration profiles. High ramp rates are certainly useful in order to form shallow and highly active layers. Laser annealing represents the most promising approach to match the requirements for the needs of fabrication of the future devices. The feasibility of laser annealed ultra-shallow junctions, with depths below 100 nm and high electrical activation, is demonstrated.