Diffusion is the process of doping the semiconductor either p-type or n-type. In this process boron is used as the p-type dopant which is available with gaseous source diborane. Also, phosphorus or arsenic is used as the n type dopant which is available with gaseous source i.e. phosphine or arsine. For each of these sources a dedicated diffusion furnace is used then the temperature of the furnace is raised about 800-1000Â°C and dopant sources are introduced in their gaseous phase. Pyrolytic decomposition of these compounds takes place in the furnace and the dopant elements are deposited on the surface of the wafer exposed through the windows created by the photolithography process.
Initially the temperature is intentionally kept low so that the dopant atoms have low diffusivity and generally stay at the surface forming a thin sheet of dopants. This process is known as pre-deposition or pre-dep in short. Then the source of the dopant is shut-off and the temperature of the furnace is raised to 1000-1400Â°C. This rise in temperature causes increase in the diffusivity and the dopant atoms deposited on the surface now diffuse deeper into the substrate this process is known as drive-in.
The pre-dep process is also known as the constant source diffusion because the concentration of source dopant atoms is maintained constant during this process. On the other hand the drive-in process is known as limited source diffusion because no extra dopant atoms are added during this process.