Polysilicon Doping Techniques

There are three (3) commonly used techniques for doping polysilicon. These are: 1) diffusion doping; 2) ion implantation; and 3) in-situ doping. Below are brief descriptions of these three polysilicon doping techniques.

Diffusion Doping:

1) Diffusion doping is generally done at a relatively high temperature (900-1000 deg C).

2) It 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.

3) The high-temperature environment of diffusion doping not only promotes dopant diffusion from the source, but also anneals the polysilicon material.

4) 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.

Ion Implantation:

1) Ion implantation deposits dopants into the poly-Si layer by directly bombarding it with high-energy ions of the dopant species.

2) Since ion implantation has destructive effects, it is followed by an annealing step that repairs the lattice disturbances and activates the implanted dopants.

3) The 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 diffusion-doped layers.

4) 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.

In-situ Doping:

1) 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 deposited.

2) In-situ doping involves the addition of dopant gases such as phosphine and diborane to the CVD reactant gases used in poly-si deposition.

3) 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.

4) Adding dopants during deposition also affects the physical properties of the poly-si layer, such as the grain size and grain orientation.

See also: Polysilicon; Polysilicon Deposition

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