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 on this page allow silicon wafers to be doped partially. The introduction of the foreign substance can be achieved using various methods: diffusion, 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Diffusion is barely used anymore for targeted doping today; it has been superseded by ion implantation.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how much heat a wafer may still be subjected to after doping, and is the reason why furnace processes have increasingly been replaced by rapid, second-scale heating.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:
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:
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.
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.
In the following processes, the wafers are placed inside a quartz tube that is heated to a specific temperature along its entire length.
A carrier gas (nitrogen, argon) is enriched with the desired dopant (likewise in gaseous form, e.g. phosphine (PH3) or diborane (B2H6)) and passed over the silicon wafers, where concentration equalization can take place.
Discs onto which the dopant has been applied are placed between the wafers. As the temperature in the quartz tube rises, the dopant diffuses out of the source discs into the atmosphere. A carrier gas then distributes the dopant evenly throughout the quartz tube, allowing it to reach the surface of the wafers.
Boron tribromide (BBr3) or phosphorus oxychloride (POCl3) serve as liquid sources. A carrier gas is passed through the liquid and thereby transports the dopant to the wafers, where it then reaches the surface of the silicon discs.
Since the entire wafer is not meant to be doped, certain regions are masked with silicon dioxide. Dopants cannot penetrate this oxide, so no doping takes place at these locations. To avoid stress or even breakage of the wafers, the tube is heated in steps (10 °C per minute) up to approximately 900 °C, at which point the dopant is introduced to the wafers. To initiate the diffusion process, the temperature is then raised to approximately 1200 °C.
In the ion implantation charged dopants (ions) are accelerated in an electric field and irradiated onto the wafer. The penetration depth can be set very precisely by reducing or increasing the voltage needed to accelerate the ions. Since the process takes place at room temperature, previously added dopants can not diffuse out. Regions that should not be doped, can be covered with a masking photoresist layer.
An implanter consists of the following components:
In contrast to diffusion processes the particles do not penetrate into the crystal due to their own movement, but because of their high velocity. Inside the crystal they are slowed down by collisions with silicon atoms. The impact causes damage to the lattice since silicon atoms are knocked from their sites, the dopants themselves are mostly placed interstitial. There, they are not electrically active, because there are no bonds with other atoms which may give rise to free charge carriers. The displaced silicon atoms must be re-installed into the crystal lattice, and the electrically inactive dopants must be activated.
Right after the implantation process, only about 5 % of the dopants are bond in the lattice. In a high temperature process at about 1000 °C, the dopants move on lattice sites. The lattice damage caused by the collisions have already been cured at about 500 °C. Since the dopants move inside the crystal during high temperature processes, these steps are carried out only for a very short time.
The substrate is present as a single crystal, and thus the silicon atoms are regularly arranged and form "channels". The dopant atoms injected via ion implantation can move parallel to these channels and are slowed only slightly, and therefore penetrate very deeply into the substrate. To prevent this, there are several possibilities:
Characteristic:
There are several types of implanters for small to medium doses of ions (1011 to 1015 ions/cm2) or for even higher doses of 1015 to 1017 ions/cm2.
The ion implantation has replaced the diffusion mostly due to its advantages.
For completeness it should be mentioned that besides ion implanation and diffusion there is an alternative process: doping using alloy. Since this procedure, however, brings disadvantages with it such as cracks in the substrate, it is not used in today's semiconductor technology any more.