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SILICON SUBSTRATES FOR SEMICONDUCTOR MANUFACTURING

SILICON SUBSTRATES FOR SEMICONDUCTOR MANUFACTURING 3.3

approximately 2.5 times the number of die to be fabricated as compared to a 200-mm wafer at a total
increase in cost, including all materials and capital, of less than 1.5 times.

3.3 THE BASICS OF SILICON WAFER MANUFACTURE

The literature is replete with information on the processes used for the manufacture of silicon wafers.
Consequently, only a brief outline of the manufacturing process will be discussed here for the sake
of completeness.
The raw material for silicon wafer manufacture is high-quality, semiconductor-grade polysilicon.
This is manufactured by reducing quartz of quartzite by carbon in arc furnaces. The resulting material,
metallurgical-grade silicon, is purified to semiconductor-grade silicon by a series of halogenation
and fractional distillation processes. Typically the silicon is converted into trichloro silane in a flu-
idized bed by reaction with anhydrous hydrogen chloride (HCl). When anhydrous HCl reacts with
metallurgical-grade silicon, a number of reaction products result. These include SiHCl , SiCl , H ,
3 4 2
and several metallic halides such as AlCl , BCl , and FeCl . Trichloro silane is fractionally distilled
3 3 3
from the impurity halides to result in a very high purity compound.
Trichloro silane is thermally decomposed into silicon and HCl in the presence of hydrogen in a
reactor in which a heated thin rod of silicon (and sometimes molybdenum) is utilized for the depo-
sition of silicon. The resulting polysilicon rods are broken up into smaller pieces to load into a cru-
cible for crystal growth.
The most widely used process of crystal growth is the Czochralski or the ingot-growth process in
which semiconductor-grade polysilicon is melted in a quartz crucible contained within a chamber
with a controlled atmosphere. A seed crystal, typically less than 1 cm in diameter, is contacted
with the silicon melt and slowly withdrawn vertically resulting in the melt in contact with the seed
solidifying into a single crystal. In this process very careful temperature control is exercised such
that the increase in the temperature of the solid-liquid interface, resulting from the latent heat of
solidification, is compensated for by a reduction in the power input to the system and hence its tem-
perature. Another important process for achieving dislocation free crystals is a process through which,
after the initial growth of a short segment of the crystal, the crystal diameter is decreased followed by
an increase in the ingot diameter. This local decrease in the diameter, called necking, causes thermal
shock induced dislocations in the growing crystal to grow out of the crystal at the neck with the sub-
sequently grown crystal being dislocation free. The final diameter of the ingot is achieved gradually
to prevent stress introduction and to control the diameter. Following the growth of an ingot of a
desired length, the pull rate is increased quickly to detach the crystal from the remaining melt in the
crucible. Figure 3.1 shows an example of a contemporary 300-mm silicon ingot.
Following ingot growth, the seed and tang ends—the ends of the ingot that do not meet the diam-
eter requirements—are cut away, the ingot is centerless ground to the exact diameter, and an orien-
tation notch (and sometimes a flat) is ground into the ingot using x-ray techniques for orientation
determination and alignment.
The ingot is now sliced into individual wafers using multiwire saws and is subsequently subjected
to a series of grinding, lapping, and polishing operations to result in the final, very high quality wafers.
Wafer fabrication operations are highly automated with an emphasis on quality with regard to
particles and wafer flatness.

3.4 SILICON AS A SUBSTRATE MATERIAL

Silicon wafers have evolved substantially over the past couple of decades. The evolution has pro-
ceeded along two directions—evolution in diameter with current state-of-the-art wafers being 300 mm
in diameter and evolution in complexity in response to the increasing demand for more functionality
of the material. Figure 3.2 is an illustration of the evolution of silicon substrates in both these attrib-
utes. In this section both these evolutionary paths will be discussed.

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