Page 462 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Inorganic Polymers 425

Even so, this level of purity falls far short of the purity need to produce the chips used in comput-
ers. The purity required is about 99.9999996 or a level of impurity of about one part in a billion.
This is achieved through multistep processes. One of these requires the silicon to be heated with
HCl at high temperatures forming the desirable volatile trichlorosilane. The vapor is condensed
then purified using distillation and absorption columns. The trichlorosilane is reacted with hydro-

o
gen gas at about 1,200 C depositing polycrystalline chip-grade silicon. The other product of this
reaction is HCl, which can be again used to create more trichlorosilane, thus eliminating the pro-
duction of unwanted byproducts.
Si + HCl → HSiCl (12.20)
3
HSiCl + H → Si + HCl (12.21)
3 2
Silicon dioxide is also used to insulate regions of the integrated circuit. Here silicon dioxide is
o
grown on the silicon surface by heating the surface to about 1,000 C in the presence of oxygen.

Si + O → SiO (12.22)
2 2
An alternate approach employs heating gaseous tetraethoxysilane to form layers of silicon
dioxide.

Si(OC H ) → SiO + 2H O + 4C H (12.23)
2 5 4 2 2 2 4

12.13 SILICON DIOXIDE IN OPTICAL FIBERS
Today, almost all telecommunication occurs via optical fibers rather than metallic wires. Signal

transmission with metallic wires was via electrons, while transmission through optical fibers is via

photons. For a rough comparison, two small optical fibers can transmit the equivalent of more than
25,000 telephone calls simultaneously or in 1 second the information equivalent to about 2 h of TV
shows. On a weight basis, 1 g of optical fiber has the transmission capability of about 300,000 grams

of copper wire. The signal loss must be small. For a typical system, the loss more than a 10 mile dis-
tance is about that found for the transmission of light through an ordinary pane of window glass.
The optical fibers connect the two longest links of a typical optical fiber communications system.

Briefly, an input signal enters an encoder generally in electrical form with the encoder transforming
it into digitized bits of 1s and 0s. This electrical signal is then converted into an optical message in
an electrical–optical converter. This converter is often a semiconductor laser that emits monochro-
matic coherent light. The message then travels through optical fibers to its target destination. Where

that distance is large, such as between countries, repeater devices are employed that amplify the
signal. Finally, at its destination, the message, in photonic form, is reconverted to an electric signal
and then decoded.
The optical fiber is formed from a combination of polymeric materials. It typically consists of

a core, cladding, and coating. The core does the actual transmission of the photons; the cladding
constrains the light so it will travel within the core with little signal power loss and little pulse dis-
tortion; and the coating helps protect the inner material from damage and external pressures. There
is a variety of materials that can be used in the construction of the optical fiber. On the basis of the

core-cladding combination, there are three types of optical systems—the step-index, graded index,
and single mode fi bers.
Most of the “long-distance” systems use high-purity silica glass as the core material. These fi bers
are thin with a thickness of the order of 5–100 mm. Containment and retention of the signal is made
possible because of the use of laser light and its total reflectance as it travels through the fi ber. This

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