5-6. Diffusion & Ion Implantation

4.2.3 Two-Step Diffusion
o A "two-step" diffusion process, which is composed of a short constant-source diffusion
followed by a limited-source diffusion, is frequently employed in reality.
- Predeposition (constant source diffusion) step:
The constant-source diffusion step to establish a known dose in a shallow layer
on the surface of the silicon. The fixed dose approximates an impulse and
serves as the impurity source for the second diffusion step.
- Drive-in step (limited source diffusion) step:
Second diffusion 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, the resulting impurity profile is closely approximated by a
Gaussian distribution, Eq.(4.6).
12