technology GaN electronics
rhombus4
Work on the substrate is currently focused on
temperatures of 1400-1600
o
C and pressures 5-6 GPa,
thinning of the silicon layer, in order to stop the
and also by CVD, which offers greater control of purity.
For MORGaN the company is developing and supplying
silicon from acting as a thermal barrier, and to
both single crystal and polycrystalline diamond substrates
produced by the CVD route, as well as some HPHT
provide a good interface for nitride deposition
single crystal. One of the key questions that MORGaN
will address is whether it is possible to deposit device
quality single crystal GaN on polycrystalline diamond.
diamond into the high-power GaN device technology is CVD is also used to make diamond-silicon composites.
still under development”. He believes that the approaches The diamond layer can have as many as three roles to
of others have strengths and weaknesses, and he is play in this type of substrate, according to Scarsbrook: it
confident that Element Six can make a significant has to act as a heat spreader; it may be needed to
contribution, because it is at the leading edge of all provide some mechanical strength, either for processing
aspects of diamond technology. In addition, Scarsbrook steps, or in the final device; and it has to provide a
suggests that there is a secondary reason for the funding backplane to the device. “This backplane is positioned on
of substrate development - it could lead to an internal the opposite side of a dielectric, and the appropriate
supplier of these products for Europe. distance – which determines the thickness of the diamond
– is around 100 µm.”
Element Six provides partners in the MORGaN project
with a variety of diamond-based substrates. These players This substrate is produced by CVD deposition of diamond
have the option of growing directly onto single crystalline on silicon. Element Six can produce 4-inch wafers by this
or poly-crystalline substrates, or depositing their epilayers route, but it is only shipping a 2-inch version to its
on a diamond-on-silicon composite. A third option is also partners in the project at present, because this size is
available – post-growth bonding of the epilayers to compatible with the majority of processing tools used in
diamond. the MORGaN effort. Work on the substrate is currently
focused on thinning of the silicon layer, in order to stop
Scarsbrook says that all three approaches face the same the silicon from acting as a thermal barrier, and to provide
challenge - managing the strain that is generated at the a good interface for nitride deposition.
interface. This arises from the significant differences in
thermal expansion coefficients of the materials involved. Putting together a good package
Element Six produces single crystal diamond by a high- If the sensors and transistors that are developed in the
temperature, high-pressure process (HPHT) that involves MORGaN project are to reach their true potential, then
they need to be housed in well-designed packages
delivering excellent thermal management capabilities.
Several partners are working towards that particular goal,
including fcubic from Sweden. This start-up uses a
combination of high-precision ink-jet printing and elevated
temperatures to produce three-dimensional packages
from a diverse range of materials, including stainless steel,
titanium, and silver. Delage says that this approach can
make relatively large packages, and it is compatible with
medium-sized markets requiring up to 10,000 pieces,
making it suitable for microwave product manufacture.
Solders used in these packages must also withstand high
temperatures. The Swedish firm Impact Coatings is active
in this area, and it has developed a new refractory metal
with a very good sheet resistance that can operate at
1000
o
C.
So it seems that MORGaN has an answer to every
potential problem associated with developing AlInN-based
devices for extreme conditions. This should serve them
Element Six can produce pieces of diamond by a well in their quest to raise the performance bar of nitride
high-pressure, high-temperature technique. sensors and transistors during the remaining two years of
CREDIT: Element Six this project.
30 www.compoundsemiconductor.net November / December 2009
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