IGBT Modules - Technologies, Driver and Application (Second Edition) - page 34

The purpose of this and other variants is to reduce the electrical field strength by
bevelling the path of the electrical field in the junction. Depending on the execution,
values near the edge structure of 80% to 95%, sometimes even 100%, can be achieved
from the 100% blocking capability of the inner part of the semiconductor. As the edge
structure does not contribute to the active surface area of the power semiconductor,
manufacturers try to keep this area as small as possiblewithout decreasing the blocking
capability of the component. Once the edge structure is processed during the
manufacturing process, the area is sealed off with a passivation layer, which may be
madeof a layer of silicondioxide (SiO
) or polyimide, amongothers.
The application of the backside metallisation is another important step. This backside
metallisationperforms several tasks:
Thermal interfacebetween the semiconductor and thepackage.
Electrical interfacebetween the semiconductor and the package.
Adhesion layer between the semiconductor and, usually, a solder layer.
The backside metallisation comprises several layers, the first of which is the adhesion
layer, which creates themechanical and electrical connection to the semiconductor. The
barrier/wet layer that follows is the actual solder layer, and creates the connection to the
solder with which the chip will later be soldered on. It also inhibits contamination of the
solder layer by preventing the diffusion of particles from thewafer or the adhesion layer.
Finally, a protective layer is applied to prevent oxidation of the barrier/wet layer prior to
the actual soldering process. The protective layer is made as thin as possible so that it
has onlyminimal influenceon the soldered joint.
Fig. 1.14
In addition to the backside metallisation, a metallic layer is applied to the front. For
IGBT, this layer is usually aluminium (Al) and is there to create a connection between
the semiconductor and the external bonding technology (bond wires). Thismetallisation
also joins together the cells that will eventually comprise a single IGBT. Alternatively, in
the future, copper (Cu)might be used tometallise the front.
As with backside metallisation, new developments aimed at achieving higher operating
temperatures and a longer cycle capability (chapter
are moving towards replacing
aluminium with copper. This entails modification of the bonding technology, which also
involves a change from aluminiumbondwires to copper bondwires (chapte
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