Page 163 - Bebop to The Boolean Boogie An Unconventional Guide to Electronics Fundamentals, Components, and Processes

144 II Chapter Fourteen
Cylindrical silicon The thickness of
/ crysta~
the wafer is determined
by the requirement for
Wafer
J 0.2 mm sufficient mechanical
A strength to allow it to
t
be handled without
damage. The actual
Figure 14-1. Creating silicon wafers thickness necessary
for the creation of the
electronic components is less than 10 pm (ten-millionths of a meter). After the
wafers have been sliced from the cylinder, they are polished to a smoothness
rivaling the finest mirrors.
The most commonly used fabrication process is optical lithography, in which
ultraviolet light (UV) is passed through a stencil-like4 object called a photo-
mask, or just mask for short. This square or rectangular mask carries patterns
formed by areas that are either transparent or opaque to ultraviolet frequencies
(similar in concept to a black and white photographic negative) and the result-
ing image is projected onto the surface of the wafer. By
means of some technical wizardry that we’ll
consider in the next section, we can
use the patterns of ultraviolet light to
50urce
grow corresponding structures in the
silicon. The simple patterns shown in
the following diagrams were selected
for reasons of clarity; in practice, a
mask can contain millions of fine
lines and geometric shapes (Figure
14-2).
Each wafer can contain hundreds
or thousands of identical integrated
circuits. The pattern projected onto
the wafer’s surface corresponds to a

Figure 14-2. The opto-lithographic 4 The term “stencil” comes from the Middle English
stepand-repeat process word stencelkd, meaning “adorned brightly.”