are the two major processes by which chemical species or
are introduced into a semiconductor such as silicon to form the
electronic structures that make integrated circuits useful (although ion
implant is now much more widely used for this purpose than thermal
Diffusion is the
movement of a chemical species from an area of
high concentration to an
concentration. The controlled diffusion of dopants into silicon,
which is accomplished using a diffusion furnace, is the foundation of forming a p-n junction and
fabrication of devices during wafer fabrication.
Diffusion is used primarily
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
mathematics of diffusion are based on
If there is a chemical concentration gradient in a finite volume of
silicon (or any matrix substance for that matter), the impurity material
will tend to move such that the
concentration gradient is
decreased. If the
flow is sustained for a sufficient amount of time, the substance will
become homogeneous and diffusion will cease.
Fick's First Law
is an equation describing the flow of an impurity in a substance,
showing that the
of material across a given plane is
across the plane.
frequently, however, Fick's First Law is not an adequate description of
the diffusion process since the
of an impurity in a finite volume of material
Fick's Second Law
describes this phenomenon. The solution to Fick's Second Law
depends greatly on the method by which the impurities are supplied to
There are two
major ways by which to deposit impurities into a substance by
In the first method, known as
a flux of impurities continuously arrives at the surface of the
substrate such that the concentration gradient of the impurity remains
constant at the surface of the substrate. In the second method,
diffusion, a thin layer of the impurity material is deposited on
the substrate. In this case, the impurity gradient at the surface
of the substrate decreases with time.
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