Page 302 - A Practical Guide from Design Planning to Manufacturing

272 Chapter Nine

Implant 1

Dopant concentration Implant 2

Depth
Figure 9-6 Ion implantation dopant profiles.

Ion implantation allows the maximum concentration of dopants to be
beneath the surface. This is particularly useful in creating wells where
a high concentration is desirable at the bottom of the well to prevent leak-
age currents, but a low concentration is needed at the surface to allow
good transistor drive current. Wells created through implantation with
concentration increasing with depth are sometimes called retrograde
wells because the concentration changes with depth in the opposite
manner as wells created through diffusion alone.
After doping, any step that heats the wafers will cause further diffu-
sion. This must be taken into account when designing the dose and
energy of the initial implant. The final distribution of dopants will be
determined by the parameters of the implant itself and the amount and
duration of all future heating of the wafer. In addition to adding dopants
into the wafer, layering steps add new materials on top of the wafer.

Deposition
Deposition steps add a new continuous layer of material across the sur-
face of the wafer. Later, patterning and etching steps may remove mate-
rial in selected regions, but initially the layer will be deposited across
the entire wafer. The four most common methods of deposition are spin-
on, sputtering, chemical vapor deposition (CVD), and electroplating.
See Fig. 9-7.
The easiest way to add a new material to a wafer is simply to pour it on.
All that is required is that the material can be made liquid or dissolved
in a liquid at a suitably low temperature and somehow later cured to be