Previously we saw how dopant atoms are deposited on a silicon wafer.
The dopant must be profiled to a desired depth in the wafer. One method
of achieving profiling is by diffusion. Profiling requires heating the
wafer and holding it at a specific temperature for a fixed period of time
so that desired diffusion depth and dopant profile are achieved.
The first step of depositing dopant is called "pre-deposition" and the subsequent profiling is called "drive-in diffusion". In pre-disposition , wafer is exposed to dopant containing gas in a high temperature environment. A constant dopant concentration is established on the exposed wafer surface. The goal of the predisposition step is to diffuse a small amount of dopant into the surface. The dopant is deposited on the surface continuously. If we now heat the wafer, the dopants will diffuse into the depth of the wafer, as well as out of the wafer. The profile of the dopant will change with time as shown in Figure 3-3.
Note that the concentration profile exhibits a sharp gradient, especially in the surface region. That is, concentration profile of dopant changes quite drastically with depth near the top of the wafer. Since introduction of each dopant atom generates a conductive species (a delocalized electron or a conduction hole), strong variations in concentration of dopant species will result in undesirable conductivity variations. Therefore, the wafer is further processed to obtain uniform dopant concentration. This is achieved by sealing the top
surface by growing a thin impervious layer of silicon oxide by oxidation. Because SiO2 layer prevents diffusion of dopant out of the top wafer surface, the dopant profile tends to be flatter in the surface region. With SiO2 sealing layer, the nature of dopant profile change is shown in Figure 3-4. Compare the two profiles. One has a region of nearly constant dopant concentration (and hence conductivity) while the other has region of variable conductivity. Profiling with a sealing SiO2 layer is called drive-in diffusion. Next, we will examine mathematical relationships which will enable us to calculate diffusion time and temperature.