Hardware Reference
In-Depth Information
Figure 3.6. Growing a pure silicon ingot in a high-pressure, high-temperature oven.
The ingot is then ground into a perfect 200mm- (8-inch) or 300mm-diameter (12-inch) cylinder, with
a small flat or notch cut on one side for handling and positioning. Each ingot is then sliced with a
high-precision saw into more than a thousand circular wafers, each less than a millimeter thick. The
wafers are then polished to a mirror-smooth surface to make them ready for imprinting. A finished
wafer with imprinted chips is shown in
Figure 3.6
.
Chips are manufactured from the wafers using a process called
photolithography
. Through this
photographic process, transistors and circuit and signal pathways are created in semiconductors by
depositing different layers of various materials on the chip, one after the other. Where two specific
circuits intersect, a transistor or switch can form.
The photolithographic process starts when an insulating layer of silicon dioxide is grown on the
wafer through a vapor deposition process. Then a coating of photoresist material is applied, and an
image of that layer of the chip is projected through a mask onto the now light-sensitive surface.
Doping
is the term that describes chemical impurities added to silicon (which is naturally a
nonconductor), creating a material with semiconductor properties. The projector uses a specially
created mask, which is essentially a negative of that layer of the chip etched in chrome on a quartz
plate. Modern processors have 20 or more layers of material deposited and partially etched away
(each requiring a mask) and up to six or more layers of metal interconnects.
As the light passes through a mask, the light is focused on the wafer surface, exposing the photoresist
with the image of that layer of the chip. Each individual chip image is called a
die
. A device called a
stepper
then moves the wafer over a little bit, and the same mask imprints another chip die
immediately next to the previous one. After the entire wafer is imprinted with a layer of material and
photoresist, a caustic solution washes away the areas where the light struck the photoresist, leaving
the mask imprints of the individual chip circuit elements and pathways. Then another layer of
semiconductor material is deposited on the wafer with more photoresist on top, and the next mask
exposes and then etches the next layer of circuitry. Using this method, the layers and components of
