Hardware Reference
In-Depth Information
semiconductor-grade rods, which are 99.999999% pure. These rods are then mechanic-
ally broken into chunks and packed into quartz crucibles, which are loaded into electric
crystal pullingovens.Therethesilicon chunksaremelted atmorethan2,500°Fahrenheit.
To prevent impurities, the ovens usually are mounted on thick concrete cubes—often on a
suspension to prevent vibration, which would damage the crystal as it forms.
After the silicon is melted, a small seed crystal is inserted into the molten silicon and
slowly rotated (see
Figure 3.4
)
. As the seed is pulled out of the molten silicon, some of
the silicon sticks to the seed and hardens in the same crystal structure as the seed. The
pulling speed (10-40 millimeters per hour) and temperature (approximately 2,500°F) are
carefullycontrolled,whichcausesthecrystaltogrowwithanarrowneckthatthenwidens
into the full desired diameter. Depending on the chips being made, each ingot is 200mm
(approximately 8 inches) or 300mm (12 inches) in diameter and more than 5 feet long,
weighing hundreds of pounds.
Figure 3.4
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,withasmallflatornotchcutononesideforhandlingandpositioning.Eachingot
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
makethemreadyforimprinting.Afinishedwaferwithimprintedchipsisshownin
Figure
