In semiconductor manufacturing diffusion of dopants occurs during any high temperature processing step. The dopant redistribution can thus be intentional or a parasitic side effect of some other thermal processes. In either event it must be controlled and monitored. Desired functions are the annealing of implanted ions already described and the introduction of dopants into the wafer from a chemical vapor source or a predeposited doped layer. Depending on the impurity concentration with respect to the intrinsic carrier concentration the diffusion process is called intrinsic or extrinsic. In the latter case the built-in potential influences the dopant movement. Other effects are the oxidation-enhanced and nitridation-retarded diffusion behavior of some dopants due to surface reactions. During an annealing step the diffusion strongly depends on the initial point defect distribution after ion implantation [Hei90], which leads to a transient enhancement. At high concentration the solubility limit of the dopant can be reached and the impurities form non-mobile clusters [Puc94] or precipitates [Lei97]. The diffusion process is then decreased and not all dopants are electrically active. The exact control of these anomalous diffusion phenomena is a very critical issue during the manufacturing of semiconductor devices.