The depth at which the ions are implanted is mainly determined by the energy and the atomic number of the ions, as well as the atomic number of the substrate material. This can be easily understood because, as an impinging ion penetrates the semiconductor, it undergoes collisions with atoms and electrical repulsion with the surrounding electrons. The distance traveled between collisions and the amount of energy lost per collision are
determined by a random process. Hence, even though all the ions are of the same type and have the same incident energy, they do not necessarily yield the same implantation depth. Instead, there is a distribution of depths represented by a standard deviation, called the straggle ΔRp. Using Fig. 2.20, the impurity concentration at a given depth x can be found if the acceleration energy EA is known: