In the previous chapter, we saw that the silicon should be changed to “N” type or “P” type by adding dopants. The addition is carried out either by diffusion process or by ion implantation process. In this section, we will learn about the diffusion process.  In general, diffusion refers to the movement of atoms or molecules. However, in semiconductor processing, the term “diffusion” usually refers to the entire process of adding a dopant to the surface of wafer at high temperature. At first, we will learn certain terminologies: What is a junction? How does diffusivity change with temperature? What are the different types of diffusions in solid substrate?  Then we will learn about the mathematical modeling of two particular cases of diffusion processes employed in IC industry. Next, we will learn about the type of equipment used for this process. Then we will learn about some of the problems associated with the processes and the ways to resolve them. Finally, we will introduce the idea of sheet resistance and its utility in IC manufacturing.

Junction: The silicon used for making the ICs is rarely without any dopant, and it usually contains a specific quantity of boron. i.e. it is  P-type silicon to begin with.  The B atoms are distributed uniformly and hence the doping profile will be uniform. Additionally during chip manufacturing process, dopants will be added by diffusion or ion implantation from the top surface of the wafer. Hence, the concentration profile of the newly added materials will not be uniform from the top of the wafer to the bottom. A schematic illustrates the same.

Figure 6.1. Schematic illustration the location of a junction

The expanded version shows that the N type dopant concentration is high on the surface and decreases as we go inside the wafer. At some location, the concentration of N and P type dopants would be equal and that is called the junction.  A more precise definition is, “the location where the concentration of electrons and holes are equal is called the junction”. In most cases, the concentration of electrons and holes are given reasonably well by the concentration of N and P type dopants respectively. If the dopants are introduced by the diffusion process, then the surface concentration of those dopants will always be higher than the concentration inside, since the atoms diffuse from high concentration region to low concentration region.