Ion Implantation and Synthesis of Materials - Studium

196 14 Application of Ion Implantation Techniques in CMOS FabricationFig. 14.2. Schematic illustration of (a) the intrusion of the source and drain depletionregions and (b) conduction band energy along the surface. The dotted line represents thelong-channel condition, and the solid line represents the short-channel condition (afterMuller and Kamins 1986)ion straggling during dopant implantation and the anomalous dopant diffusionduring thermal annealing (see Chap. 9, Sect. 5). A physical picture of these twoeffects can be obtained with the help of the modeling program Stanford UniversityProcess Engineering Modeling (SUPREM, Law et al. 1986), which accounts forion straggling and dopant diffusion during the activation annealing process.Figure 14.3a, b shows the calculated two-dimensional boron atom distributions,obtained by using SUPREM for 5 keV boron implantation through a 100 nm wideaperture, before and after subsequent annealing at 1000°C for 10 s. The maximumlateral penetration depth (measured at the boron concentration of 1 × 10 18 cm −3 ) isaround 100 nm for the as-implanted profile and around 200 nm for annealedprofiles. This anomalous lateral spreading of dopants shows the severe challengeto the device scaling. The vertical and lateral spreading of dopants can bealleviated, but not eliminated, by using ultra low energy ion implantations. Allgroup IV and group V dopants show defect-enhanced diffusion phenomena calledtransient enhanced diffusion (TED) (see Chap. 9, Sect. 5). For n-type dopants suchas As and Sb, their diffusion is mediated by interaction with vacancies. Thediffusion of the p-type dopant B, on the other hand, is mediated by the interactionwith Si self interstitials. As and Sb have larger masses compared with B. Theheavier mass more easily creates an amorphous substrate layer during ionimplantations. Diffusivities of As and Sb are also slower than that of B. Therefore,the formation of B-doped p + ultra shallow junctions is more challenging than thatof n + shallow junctions.