Atomic diffusion or atomic rearrangement in thin films is the basic kinetic process in microelectronic device manufacturing and reliability. Pure Si is not useful until we can diffuse electrically active n-type and p-type do pants into it. In fact, the fundamental behavior of a transistor, i.e. the p–n junction in silicon, is obtained by a non-uniform distribution of both n- and p-type do pants in Si in order to achieve the built-in potential which guides the motion of electrons and holes in the transistor. Thus, the diffusion of do pants in Si has been a very important subject in microelectronics, both in device characteristics and in device manufacturing. Indeed, there are some very sophisticated programs to simulate and to analyze the do pant diffusion profile in junction formation in Si devices.
In classical metallurgy, a blacksmith inserts a bar of iron into a charcoal furnace to allow the gas phase of carbon to diffuse into the iron. The diffusion time is typically short, just several minutes' heating in the furnace, so the blacksmith has to take out the red-hot bar and hammer it in order to homogenize the carbon in the bar. This process of “heat and beat” is to diffuse and to redistribute carbon in iron to make the Fe–C alloy.
In this chapter, we shall connect microscopic atomic jumps in a crystalline lattice to macroscopic diffusion behavior as described by the Fick's first and second laws.