3.5 Ion-Implantation and Diffusion

Diffusion and Ion Implantation are the methods by which impurity is introduced into silicon to change its resistivity. These processes allow the formation of sources and drains of MOSFETs and active regions of bipolar transistors. In diffusion (in a high temperature environment) the dopant atoms are moving from a highly concentrated region to a low concentrated region. The classic approach to creating regions of silicon with different electrical properties was to deposit a dopant material, on the surface, then diffuse or drive it into the surface of the silicon by exposing it to controlled periods of high temperature. As device geometries have become smaller, the side-ways diffusion has become more difficult to deal with, so the industry has converted to the ion implantation process. In implant, the dopant molecules are implanted vertically into the surface of the silicon by a high-energy ion beam. This penetrates the silicon vertically without any appreciable side-ways diffusion. Thus p-type or n-type regions in silicon are created when they are doped with Boron and Phosphorous ions respectively. The following section will deal with diffusion of dopants in a Si lattice.