Page 299 - A Practical Guide from Design Planning to Manufacturing

Semiconductor Manufacturing 269

Tube furnace
AsH gas
3
H gas
2
SiO 2 SiO 2
As N+
diffusion
region
Figure 9-3 Solid state diffusion.
Si wafer

region. This is true only because there are more molecules inside the
region than outside. This causes regions of high concentration to diffuse
into regions of low concentration. Once the concentration is uniform
everywhere, the chances of a molecule of ink leaving or entering any
region is the same, and there is no longer any net diffusion. Diffusion will
always occur when there is a nonuniform concentration of material and
there is sufficient energy for it to move inside the medium. The concen-
tration of ink in the glass could be fixed in some nonuniform distribu-
tion by quickly freezing the water before uniform concentration could be
reached. Heating the water would cause the rate of diffusion to increase.
Heating solids allows diffusion to occur within them as well. At 800°C
to 1200°C, many different dopant atoms will readily diffuse through sil-
icon. This allows doped regions to be created within a silicon wafer by
heating it and exposing it to a gas containing the desired dopant.
Figure 9-3 shows solid-state diffusion forming an N-type region in a
) grown
silicon wafer. The wafer first has a layer of silicon dioxide (SiO 2
upon it and is then patterned so that holes exist where N-type regions
are needed. The wafer is heated in a furnace where a gas breaks down
at the surface of the wafer to release dopant atoms. These diffuse through
the silicon, gradually moving from regions of high concentration to low con-
centration. The speed at which diffusion occurs depends upon the tem-
perature and the type of dopant being used. Common N-type dopants
include arsenic (As), phosphorous (P), and antimony (Sb). The only effective
P-type dopant for silicon is boron (B).
Solid-state diffusion is simple but has some serious limitations on
the types of dopant profiles that can be created. The highest concen-
tration of dopants will always be at the surface of the wafer. Allowing
diffusion to occur for a longer time or at a higher temperature will
make the change in concentration with depth more gradual, but the
peak concentration will always be at the surface. Figure 9-4 shows some
dopant profiles possible through solid-state diffusion.