IGBT Modules - Technologies, Driver and Application (Second Edition) - page 30

1.1.7 Manufacturingprocess
Monocrystallinewafers are required for themanufacture of semiconductors. The source
material for the latest power semiconductors such as thyristors and IGBTs is the
element silicon (Si). Silicon occursmost often in nature in connectionwith quartz (SiO
for example. Pure silicon is obtained through an electrothermal process that separates
the bound oxygen from the silicon by oxygenation using carbon. The raw silicon
obtained in thismanner is then subjected to a series of purification processes to reduce
the proportion of impurities in the material, which can be as high as 3%, to a value
below0.00001%. There are several phases to thepurificationprocess:
First the conversion of the raw silicon to liquid trichlorosilane (SiHCl
) through a reaction
with hydrochloric acid (HCl) and subsequent distillation is done. The next step is to
introduce a gas mixture comprising trichlorosilane and additional hydrogen (H
) into a
reaction chamber, intowhich rods of silicon are placed. These silicon rods are heated to
approximately 1200°C. At this temperature, the trichlorosilane reacts with the hydrogen
and decomposes back into hydrochloric acid and silicon. The silicon is deposited on the
rods in polycrystalline formand the rods grow in diameter to30cm (current technology).
In the next process step, the polycrystalline silicon is transformed into a single crystal,
using Czochralski's crucible process or a floating-zone (FZ) process. In the CZ or
Czochralski process
the purified raw silicon is melted in a crucible made of quartz,
graphite or a similarmaterial. The temperature of themelt is just above themelting point
of the source material. A seed crystal, applied to a revolvable rod, is brought as far as
the surface of themelt. Due to the good thermal conductivity of the seed crystal and the
rod connected to it, thematerial in the contact zone cools to a value below the melting
point and deposits at the seed crystal. The slow turning of the rod (0.3 to 1.3 revolutions
per second) and a simultaneous upwardmovement (2 to 250
) makes it possible to
pull a single crystal from themelt. The diameter of the crystal depends on the speed of
the upwardmovement. In theCZ process, the silicon touches the crucible, so the single
crystal is slightly impure. The use of single crystals obtained by the CZ process is
therefore limited to epitaxial wafers in the fieldof power electronics.
In the FZ process, the purified silicon rod is hung vertically. At the other end, a slowly
revolving seed crystal is moved toward the rod without touching the rod. Finally, the
lower part of the silicon rod is heated. This causes the end to bulge and touch the seed
crystal, which thus begins to grow. The melting zone is moved progressively upward
(approximately 10 to 20
) according to the speed of growth of themono-crystal. The
FZ process is usually used to manufacture ultra-pure wafers. Substrates manufactured
using theFZprocess are therefore used tomakeNPTor Trench-FS IGBT (chapte
Named after thePolish scientist JanCzochralski (1885 - 1953).
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