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.  


Fig. 1. Examples of Ion Implanters


As mentioned, every implanted ion collides with several target atoms before it comes to a rest.  Such collisions may involve the nucleus of the target atom or one of its electrons.   The total power of a target to stop an ion, or its total stopping power S, is the sum of the stopping power of the nucleus and the stopping power of the electron.  Stopping power is described as the energy loss of the ion per unit path length of the ion.


The damage caused by atomic collisions during ion implantation changes the electrical characteristics of the target.  Many target atoms are displaced, creating deep electron and hole traps which capture mobile carriers and increase resistivity.   Annealing is therefore needed to repair the lattice damage and put dopant atoms in substitutional sites where they can be electrically active again.  


Wafer Fab Links:  Incoming Wafers Epitaxy Diffusion Ion Implant Polysilicon Dielectric Lithography/Etch Thin Films Metallization Glassivation Probe/Trim


See Also:  Implant Annealing IC ManufacturingWafer Fab Equipment




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