are three (3) commonly used techniques for
These are: 1) diffusion doping; 2) ion implantation; and 3) in-situ
doping. Below are brief descriptions of these three polysilicon doping
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doping is generally done at a relatively high temperature (900-1000 deg
involves the growing or deposition of a highly-doped glass on the
undoped polysilicon. This doped glass will serve as the source of
dopants that will diffuse into the polysilicon material.
high-temperature environment of diffusion doping not only promotes
dopant diffusion from the source, but also anneals the polysilicon
Diffusion doping's advantage is its ability to introduce very high
concentrations of dopants into the poly-Si layer, attaining low levels
of resistivity. The high processing temperature and its tendency to
increase surface roughness are its drawbacks.
the poly-Si layer by directly
2) Since ion
implantation has destructive effects, it is followed by an annealing
step that repairs the lattice disturbances and activates the implanted
advantage of ion implantation is its ability to control dopant dosage
with high precision. However, it can not attain the low
resistivities achievable by diffusion doping, i.e., even heavily doped
ion-implanted poly-Si layers exhibit 10X the resisitivity exhibited by
Ion-implanted polysilicon layers are often used in applications where
high conductivity is not required, such as being employed as high-value
load resistors in circuits.
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In-situ doping refers to the doping technique wherein the dopants are
introduced to the poly-si at the same time the poly-si layer is being
In-situ doping involves the addition of dopant gases such as phosphine
and diborane to the CVD
reactant gases used in poly-si deposition.
In-situ doping is not a simple process, since the introduction of the
dopant gases complicates the control of layer thickness, dopant
uniformity, and deposition rate.
Adding dopants during deposition also affects the physical properties of
the poly-si layer, such as the grain size and grain orientation.
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