Predictions of range distributions

An essential first step in the consideration of ion implantation effects is to understand how energy is coupled into the target material. We will first present examples of energy transfer and ion range, and then indicate how these features have been calculated. In practice there has been a continuous interaction between the theoretical and experimental assessments of ion ranges. This has resulted in modifications to the theories so that there are now tabulations and computer codes which predict ion ranges in virtually any ion/target combination. These computations are accurate to within 5–15%. Consequently, although it is useful to know the underlying assumptions of the range theories, and hence their limitations, the majority of the profiles for the distributions of implanted ions are calculated from standard computer simulations. Since knowledge of the ion range, damage distribution or surface sputtering involves many factors in addition to the initial ion range, the existing level of accuracy is perfectly acceptable. Indeed, divergence between measured and computed ranges is frequently not a result of a failure of the computation, but, rather, it results from the fact that such computer codes do not allow for subsequent migration and secondary processes. As has already been mentioned briefly in Chapter 1, there are two main processes which slow down the incoming ion. These are electronic excitations and nuclear collisions. The rate of energy transfer for each process is a function of the nuclear charge and mass of the incoming ion (Z1, M1), and the target (Z2, M2), as well as the energy.