Ion Implantation and Synthesis of Materials - Studium

122 9 Doping, Diffusion and Defects in Ion-Implanted Sidissociative reaction of a substitutional impurity atom (S) into an interstitialimpurity atom (I) and a vacancy (V):S ⇔ I + VThe movement of impurities becomes primarily controlled by the rate of thisdissociation.9.4.4 Interstitialcy and the Kick-Out MechanismWhen the diffusing interstitial atoms have a size comparable to that of the latticeatoms, the interstitialcy mechanism for diffusion may take place. In this case theinterstitial impurity atom moves into a host lattice site by pushing a neighboringnormal atom into the adjacent interstitial site. This process repeats itself when aself-interstitial atom pushes the substitutionally located impurity atom into aninterstitial site.The kick-out mechanism is rather similar to the interstitialcy mechanism. Inthis case, a host self-interstitial atom diffuses around the lattice. When it reaches asubstitutional impurity atom, the self-interstitial pushes the impurity atom into anadjacent interstitial site. The interstitial impurity then diffuses interstitially until itreverts back to a host lattice site by displacing a host atom. It is experimentallydifficult to distinguish the kick-out mechanism from the interstitialcy mechanism.The generally accepted view is that the interstitial impurity atoms may tend todiffuse longer distances before returning to the normal lattice sites for the kick-outmechanism, whereas the impurity atoms tend to diffuse interstitially for arelatively short distance before going into the normal lattice sites for theinterstitialcy mechanism.9.5 Transient Enhanced Diffusion of BoronIntegrated circuit technology requires a reduction in device dimensions. Thejunction depth, where the donor and acceptor concentrations are equal, is setbetween 10 and 30 nm. These shallow junction requirements restrict ionimplantation technology – not on the implantation procedures themselves, but onthe subsequent diffusion of the implanted species during thermal annealing.Figure 9.12 shows boron depth profiles for a B dose of 1015 cm −2 at animplantation energy of 0.5 keV and thermal heat treatment at 1,050°C for 10 s.The boron depth profile for 0.5 keV implants is consistent with shallow junctionrequirements of junction depths of ~20 nm. After thermal annealing, the boronprofile has spread to depths of 100 nm or more – well beyond the shallow junction