Callister - An introduction - 8th edition

138 • Chapter 5 / Diffusion
millions of interconnected electronic devices and circuits are embedded in one of the
chip faces. Single-crystal silicon is the base material for most ICs. In order for these IC
devices to function satisfactorily, very precise concentrations of an impurity (or impurities)
must be incorporated into minute spatial regions in a very intricate and detailed
pattern on the silicon chip; one way this is accomplished is by atomic diffusion.
Normally two heat treatments are used in this process. In the first, or predeposition
step, impurity atoms are diffused into the silicon, often from a gas phase, the
partial pressure of which is maintained constant. Thus, the surface composition of
the impurity also remains constant over time, such that impurity concentration
within the silicon is a function of position and time according to Equation 5.5. Predeposition
treatments are normally carried out within the temperature range of 900
and 1000C and for times typically less than one hour.
The second treatment, sometimes called drive-in diffusion, is used to transport
impurity atoms farther into the silicon in order to provide a more suitable concentration
distribution without increasing the overall impurity content. This treatment
is carried out at a higher temperature than the predeposition one (up to about
1200C), and also in an oxidizing atmosphere so as to form an oxide layer on the
surface. Diffusion rates through this SiO 2 layer are relatively slow, such that very
few impurity atoms diffuse out of and escape from the silicon. Schematic concentration
profiles taken at three different times for this diffusion situation are shown
in Figure 5.9; these profiles may be compared and contrasted to those in Figure 5.5
for the case wherein the surface concentration of diffusing species is held constant.
In addition, Figure 5.10 compares (schematically) concentration profiles for predeposition
and drive-in treatments.
If we assume that the impurity atoms introduced during the predeposition treatment
are confined to a very thin layer at the surface of the silicon (which, of course,
Concentration of diffusing species
Concentration of diffusing species (C)
C s
x j
profiles taken after (1) predeposition and (2)
Figure 5.9 Schematic concentration profiles
for drive-in diffusion of semiconductors at
t 1
t 3 > t 2 > t 1
three different times, t 1 , t 2 , and t 3 .
t 2
Distance
Figure 5.10 Schematic concentration
drive-in diffusion treatments for
After predeposition
semiconductors. Also shown is the junction
depth, x j .
C B
After drive-in
t 3
Distance into silicon (x)