1.1.6 Ion Implantation

The ion implantation process is a technique by which dopants are introduced into the semiconductor. Ions of a desired material are accelerated via an electric field to impact a solid surface, in our context the silicon wafer. Dopant ions such as boron, phosphorus, or arsenic are generally created from a gas source, for purity reasons, prior to their acceleration towards the silicon interface, where they penetrate into the silicon crystalline lattice. After the penetration into the top layers of the lattice, an annealing step is performed, resulting in the generation of a charge carrier in the semiconductor for each dopant atom in the lattice. The generated charge carrier can be a hole or an electron depending on whether the dopant used is of p-type or n-type, respectively. An alternative doping method for semiconductors is the diffusion of dopants into the material. Some advantages of ion implantation over diffusion, as described next, are the short process times, good homogeneity and reproducibility, relatively low temperatures required during processing, a range of materials being available as masks (oxide, nitride, metals, resist, etc.), and the ability to implant thin layers, resulting in surfaces with high dopant concentration gradients.