Page 82 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

WAFER PREPARATION 63

4.2 WAFER PREPARATION
4.2.1 Crystal Growth

As stated in the preceding text, silicon is the most commonly used semiconductor mate-
rial for making microelectronic devices. Other semiconductor materials are employed in
certain important niche areas, for example, gallium arsenide (GaAs) for optoelectronic
devices and silicon carbide and gallium nitride (GaN) for high-power or high-temperature
devices, but silicon is by far the dominant material not only for standard microelectronic
components but also for microtransducers and MEMS devices.
Silicon is one of the earth's most abundant elements forming about 25 percent of
its surface crust. However, silicon reacts readily with oxygen and other materials in the
earth's atmosphere and hence is generally found in the form of sand (i.e. silicon dioxide
(SiO 2)) or silicates. Sand can be found with an impurity level of less than 1 percent,
and this composition is usually the starting point for making single-crystal silicon wafers.
The process has three main steps: first, refining the sand into polycrystailine silicon rods;
second, growing the single-crystal silicon rods, known as boules; and third, producing the
wafers suitable for monolithic processing (see Section 4.3).
The first step is achieved by placing this relatively pure form of sand, quartzite, into
a furnace with a common source, such as coal, coke, or wood chips, of carbon. The
silicon dioxide (SiO 2) is reduced by carbon and the condensed silicon vapour to form
metallurgical-grade silicon (98 to 99 percent pure):

SiO 2(s) + 2C(s) Si(s) + 2CO(g) (4.1)

Next, the metallurgical-grade silicon is further purified by heating it up in an atmosphere
of hydrogen chloride (HC1) to form the compound trichlorosilane (SiHCl 3):

Si(s) + 3HCl(g) SiHCl 3(g) + H 2(g) (4.2)

The trichlorosilane is cooled to form a liquid at room temperature (boiling point is
32 °C) and is easily purified to semiconductor standards by a fractional distillation proce-
dure. The distillation procedure removes the unwanted chlorides of metallic and dopant
impurities, such as copper, iron, phosphorous, and boron. Finally, the thermal reduc-
tion of trichlorosilane gas in a chemical vapour deposition (CVD) reactor produces
electronic-grade polycrystailine silicon, which is deposited onto a slim pure silicon rod
(see Figure 4.2).
SiHCl 3(g) + H 2(g) Si(s) + 3HCl(g) (4.3)

The electronic-grade silicon rods are typically 20 cm in diameter and several metres
in length, and these are used to grow single-crystalline silicon by either the Czochralski
or the zone-melt process.
The present-day Czochralski process is based on the process invented by Czochralski in
1917 for the growth of single crystals of metals. It consists of dipping a single-crystal seed