a chemical species into a substrate by direct
bombardment of the substrate with
high-energy ions of the
chemical for deposition.
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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
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
implantation, impurity atoms are
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
treatment is used to
or repair the crystal lattice disturbances caused by the atomic
1. Examples of Ion Implanters
every implanted ion
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.
is described as the energy loss of the ion per unit path length of the
damage caused by atomic collisions during ion implantation
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.
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