Page 17 - Power Electronics Handbook

10 Power semiconductor devices

way to the boat containing the silicon slices. The furnace temperature is
kept close to the melting point of silicon, Le. 12WC, and at this value the
silicon atoms are highly mobile. Impurity atoms readily move through the
silicon lattice by substitution, going from a region of high concentration to
one of lower density.
Diffusion can be carried out by one of two techniques, as shown in
Figure 1.2. In the error function or one-step process the concentration of
impurities is kept ked throughout the diffusion period, giving the curves
shown. In Gaussian or two-step diffusion a fixed amount of impurity is
present, so that as this moves deeper into the silicon bulk the surface
concentration decreases. This gives a flatter dopant distribution of higher
resistivity. Generally, diffusion takes place in a slightly oxidisiig
atmosphere. This results in the formation of a glassy layer of impurity on
the silicon surface, as well as a slight penetration into the silicon bulk.
The silicon slice can be removed to a second furnace and heated in an
inert atmosphere, when the dopants diffuse out of the glassy layer to give
an error function distribution. The glassy layer now not only forms a
diffusion source but also protects the silicon surface from evaporation, and
acts as a getter for impurities from the silicon bulk. For Gaussian diffusion
the glassy layer is etched off using hydrofluoric acid, prior to the slice being
heated in an inert atmosphere. The critical dopants just below the surface
now diffuse into the silicon bulk.
Apart from the open-tube arrangement, shown in Figure 1.2(a), it is
possible to diffuse slices by putting them in a sealed quartz container with
doped silicon powder and then heating the combination in a quartz
furnace. The advantage of this method is that many slices can be diffused
simultaneously, giving a larger throughput. However, the quartz container
must be broken to remove the slices after diffusion, so the process can
prove expensive.
The most commonly used p- and n-type impurities are boron and
phosphorus respectively. Both reach maximum solubility at about 1200°C
and have a high diffusion constant. Arsenic, on the other hand, is an n-type
impurity, which diffuses very slowly. It is used for making the buried layer
in transistors, since this must not diffuse appreciably during subsequent
high-temperature processes.
Gold is often introduced as an impurity into semiconductor devices. This
is a lifetime killer and enables fast-switching components to be built. The
gold atom is much smaller than a silicon atom. This means that it does not
move through the silicon lattice by substitution, as other impurities do, but
very rapidly in between the silicon atoms, that is, intrinsically.

1.2.6 Ion implantation
Ion implantation is a technique for precisely determining the concentration
and location of impurities within a silicon slice (Singer, 1995b). In this
process selected ions of the required impurity are accelerated into the slice of
silicon, where they plough their way through the crystal structure to the
required depth. Because of this the material is distorted after ion implantation
and it usually needs to go through an annealing stage, in which the atoms are
allowd to drift into their places within the lattice.