IGBT Modules - Technologies, Driver and Application (Second Edition) - page 32

The process of neutron irradiation is based on the fact that natural silicon is found in
different isotopes
Si with a proportion of 92.2%,
Si with a proportion of 4.7% and
Si with a proportion of 3.1%. When irradiated with neutrons,
Si converts, with the
addition of a
quantum, to the unstable isotope
Si decays, with a half-life of
2.63h, into the stable phosphoric isotope
P, releasing one electron (
decay). The
phosphorus created in this way from the fifth group in the periodic table now functions
as adonor.
After the mono-crystal is manufactured in the form of silicon rods, the rods are cut into
slices. Finally, the surface undergoes a variety of process steps such as grinding and
surface refinement. The end product is a silicon wafer that serves as the substrate for
the relevant power semiconductor.
Wafers come in different sizes. It is generally desirable that the wafers be as large as
possible, as this increases the yield of finished semiconductors and thus reduces the
costs per item. However, making a wafer larger places greater demands on the
precision of themanufacturing process. It makes sense to increase the diameter of the
wafer only when the process is accurate enough and repeatable. Until 2007, for
example, IGBTs were beingmanufactured with a diameter of 6 inches (actual diameter
150mm), but since 2008, more and more manufacturers have begun using 8-inch
wafers (actual diameter 200mm).
Depending on the substrate, the individual layers with their different doping and
structures are applied in further process steps. This involves a variety of processes,
Epitaxy involves applying layers of silicon, with or without dopingmaterials, toa
source material created using the CZ process. Firm silicon layers (as well as
any doping materials present) of great purity are separated from gaseous
silicon compounds at temperatures in the regionof 1000°C to1200°C.
Implantation involves putting doping material into the source material by
targeted bombardment of the material with ions in an ultra-high vacuum
process. The concentration profile of the semiconductor can be controlled by
the accelerationenergy and the concentrationof ions.
Diffusion mixes the source material with a doping material. The process is
carried out ina special diffusion furnaceat temperatures of around1000°C.
To achieve a controlled structure with the above implantation or diffusion processes,
masksmust be applied to the sourcematerial.
hows a simplified form of this
process. After a thin layer of insulation (usually SiO
) and a light-sensitive photo-resist
lacquer have been applied, the exposure and subsequent development are carried out.
After the development, an etching process removes the insulation layer from the
Atoms of an element are determined by the number of protons they have in their atomic nucleus. Atoms that
have the same number of protons but a different number of neutrons are known as isotopes. In general, every
element has several isotopes, some of which may be stable and others unstable (radioactive). Due to the
different number of neutrons, isotopes are differentiated according to their mass number (the number of nucleons
they have), which is the same as the sum of all the protons and neutrons in their atomic nucleus. The mass
number is aprefix to theelement symbol.
1...,22,23,24,25,26,27,28,29,30,31 33,34,35,36,37,38,39,40,41,42,...548
Powered by FlippingBook