Ion Implantation

Ion implantation is the process of depositing a chemical species into a substrate by direct bombardment of the substrate with high-energy ions of the chemical for deposition. 

Over the years, ion implant has steadily replaced thermal diffusion for doping a material in wafer fabrication because of its many advantages. The greatest advantage of ion implant over diffusion is its more precise control  for depositing  dopant atoms into the substrate.

Doping, which is the primary purpose of ion implant, is used to alter the type and level of conductivity of semiconductor materials.  It is used to form bases, emitters, and resistors in bipolar devices, as well as drains and sources in MOS devices. It is also used to dope polysilicon layers.

During ion implantation, impurity atoms are vaporized and accelerated toward the silicon substrate.  These high-energy atoms enter the crystal lattice and lose their energy by colliding with some silicon atoms before finally coming to rest at some depth.  Adjusting the acceleration energy controls the average depth of depositing the impurity atoms. Heat treatment is used to anneal or repair the crystal lattice disturbances caused by the atomic collisions.