Short
constant-source diffusion is often followed by limited-source diffusion,
resulting in a "two-step" diffusion process. [1]

The
constant-source diffusion step is used to de a known dose in a shallow layer on
the surface of the silicon and is called the **predeposition **step. The fixed dose approximates an
impulse and serves as the impurity source for the second diffusion step.

The
second diffusion is called the drive-in step and is used to move the diffusion
front to the desired depth. If the Dt product for the drive-in step is much
greater than the Dt product for the predeposition step, a Gaussian distribution
closely approximates the resulting impurity profile and is given by the
following expression.

_{}

** **