Chapter 4 Diffusion

Diffusion is the transport mechanism by which dopant atoms are distributed in the silicon crystal to the derived depth and desired concentration. In an earlier section we introduced dopant atoms on the surface of the wafer using a vapor deposition method (Fig.3-1a) or by ion implantation technique (Fig 3-1b).

Figure 3-1. Two Geometry's of Dopant Diffusion.

You will note that in the case of CVD method, the SiO2 layer is removed prior to CVD deposition while in ion implantation method, dopant atoms are implanted through the oxide layer. In the diffusion step, that follows, profiling the dopant concentration and its profile are accomplished. Ideally the dopant will penetrate to a desired distance, called junction depth, Xj below the wafer surface and will be of uniform concentration. In this chapter you will be first introduced to physics of diffusion in solid state, followed by the development of a diffusion model to describe evolution of dopant concentration profile in a thermal diffusion process.