Diffusion

Doping means introducing a foreign substance into the semiconductor crystal in order to deliberately alter its conductivity through an excess or a deficiency of electrons. In contrast to doping during wafer fabrication itself, where the entire wafer is doped, the doping techniques described in this section allow silicon wafers to be doped partially. The introduction of the foreign substance can be achieved using various methods: diffusion, ion implantation (and alloying).

Diffusion is barely used anymore for targeted doping today; it has been superseded by ion implantation. Nevertheless, understanding it remains essential, because it never stops acting: every subsequent high-temperature step causes dopants that have already been introduced to keep migrating. This limits how much heat a wafer may still be subjected to after doping, and is the reason why furnace processes have increasingly been replaced by rapid, second-scale heating.

Diffusion means the spontaneous spreading of a substance within another substance due to a difference in concentration; for example, a drop of ink in a glass of water becomes evenly distributed after a certain amount of time. In a silicon crystal, one finds a solid lattice of atoms through which the dopant must move. This can happen in three ways:

• Vacancy diffusion: the foreign atoms occupy empty sites in the crystal lattice, which can always occur. Similar to hole conduction, an interplay arises between occupied lattice sites and vacancies.
• Interstitial diffusion: the foreign atoms move between the silicon atoms within the crystal lattice.
• Site exchange: foreign atoms located in the crystal lattice swap lattice sites with silicon atoms.

The dopant can continue to move within the semiconductor crystal until either a concentration gradient has been equalized, or the temperature has been lowered far enough that the atoms can no longer move.
The speed of the diffusion process depends on several factors:

• dopant
• concentration difference
• temperature
• substrate
• crystal orientation of the substrate (see fabrication of single crystals)

Diffusion with an Exhaustible Source

Diffusion with an exhaustible source means that only a limited amount of dopant is available. The longer the diffusion process continues, the lower the concentration at the surface becomes; in return, the penetration depth into the substrate increases. The diffusion coefficient of a substance indicates how quickly it moves within the crystal. Arsenic, with a low diffusion coefficient, penetrates the substrate more slowly than, for example, phosphorus or boron.

Diffusion with an Inexhaustible Source

Here, the dopant is available in unlimited quantity. The concentration at the surface therefore remains constant throughout the process, since particles that have penetrated into the substrate are continuously replenished.