1.1.7 Diffusion

The movement of doping atoms according to a dopant gradient in a semiconductor material is called diffusion. It occurs during any high temperature processing step, either as an intentional or as a parasitic effect. Due to the requirement of very shallow junctions in modern semiconductor technology diffusion is mainly a parasitic effect of the annealing step after ion implantation or of an oxidation step which is performed at high temperatures. Nevertheless there are still applications in the well formation in complimentary metal oxide semiconductor (CMOS) technologies or in the in-diffusion of dopants from a chemical vapor source.

There are strong variations in the diffusivity of different dopant species and the diffusivity can depend on the concentration of atoms of the own and other species. Besides, diffusion can be enhanced by oxidation or retarded by nitridation, because these processes generate point defects at the surface. These surface generated point defects as well as implantation induced point defects can have a strong influence on the diffusivity because they facilitate complex diffusion mechanisms like transient enhanced diffusion (TED). At high concentration levels the dopants can form non-mobile clusters or precipitates which decreases the average diffusivity. The exact control of all diffusion mechanisms is a very critical issue during the manufacturing of a semiconductor device, because each redistribution of the dopants significantly influences the electrical characteristics.