We now put the wafer in a sputter
deposition system. In the sputter system, we coat the
entire surface of the wafer with a conductor. An
aluminum-silicon alloy is usually used, although other metals
are employed as well.
A sputtering system is shown schematically in
Figure 1. A sputtering system is a vacuum chamber,
which after it is pumped out, is re-filled with a low-pressure
argon gas. A high voltage ionizes the gas, and creates what is
known as the Crookes dark space near the cathode,
which in our case, consists of a metal target made out of the
metal we want to deposit. Almost all of the potential of the
high-voltage supply appears across the dark space. (The glow
discharge consists of argon ions and electrons which have been
stripped off of them. Since there are about equal number of ions
and electrons, the net charge density is about zero, and hence
by Gauss' law, so is the field.)
The electric field accelerates the argon atoms which slam into
the aluminum target. There is an exchange of momentum, and an
aluminum atom is ejected from the target (
Figure 2)
and heads to the silicon wafer, where it sticks, and builds up a
metal film
Figure 3.
If you look at
Figure 3, you will note that we have
seemingly done something pretty stupid. We have wired all of the
elements of our CMOS inverter together! Ah, but all is not
lost. We can do one more photolithographic step, and pattern and
etch the aluminum, so we only have it where we need it. This is
shown in
Figure 4.
"This course offers an introduction to solid state device including field effect and bipolar transistors. Properties of transmission lines and propagating E&M waves are also presented. It is […]"