2.7 Ion Implantation

Ion implantation is the most widely used method to introduce dopant impurities in semiconductors. In this method a beam of ionized impurity atoms is accelerated through an electrostatic field and aimed at the semiconductor target. The ions penetrate the semiconductor loosing their energy in the collisions with the target atoms until they come to rest. The distance traveled, or the penetration depth, is a function of the kinetic energy of the ions. As this can be precisely controlled (via the electrostatic field), ion implantation allows the formation of vertically shallow dopant profiles (very important in modern technologies). The dose is also tightly controlled via the ion current. This allows a fine tuning of the dopants profile and makes ion implantation a high reproducible process.

During their trajectory in the semiconductor, ions collide with lattice atoms that become dislodged. This causes implantation damage. Another problem arises as the new dopant atoms reside in interstitial sites of the crystal lattice which are electrically inactive. To reestablish the crystalline structure it is necessary to submit the wafer to a thermal treatment. However, this process limits the resolution of an implanted profile and special annealing techniques which cause small movement of the dopants, such as rapid thermal annealing, are required. Another problem is channeling which occurs when implanting into single crystal materials where due to the anisotropy of the target fewer collisions occur and the penetration depth is increased. To overcome these phenomena the ion beam must be slightly tilted in relation to the crystal's prefered orientations.