MOCVD technology
rhombus4
Interestingly enough, MOCVD growth platforms for these Fig. 4:
much larger wafers do already exist. The CRIUS Close Temperature
Coupled Showerhead Reactor is one of the widely used profile of a
reactor types for LED manufacturing, and although today 200 mm
it is usually run in a 31x2-inch configuration, the geometry GaN/AlGaN/Si
of this reactor can be easily converted into a single 200 wafer measured
mm or even a 300 mm setup. The 200 mm setup was by ARGUS.
used to perform GaN LED growth experiments. Thickness The red dots
uniformity of less than 1.9% (std. deviation) was achieved indicate
on a 6-inch wafer (Fig 3; 6 mm rim excluded). susceptor area
without wafer;
Delivering GaN processes on 200 mm substrates hints at the blue dots
another potential route to cost efficient manufacturing - represent wafer
the use of silicon substrates. Silicon is available at a much area
lower price level compared to sapphire, and is readily
available at a commercially attractive price up to 300 mm
in diameter. Consequently, GaN-on-silicon processes
have been investigated for quite some time.
along the susceptor radius. As the susceptor rotates, a
Although the target applications during the last few years thermal image of the entire wafer surface (and the
were GaN based electronic devices rather than LEDs, in suceptor area beyond the wafer) is obtained (Fig. 4). This
2008, a partnership between IMEC (Belgium) and temperature map is finally used to optimize growth
AIXTRON resulted in a successful demonstration of 200 conditions.
mm GaN/AlGaN MOCVD growth on (111)-oriented silicon
(reported at IWNS 2008). The growth results clearly show that GaN/Si processes
can be run in a stable and well controlled manner with
To achieve this result, a variety of fundamental physical excellent uniformities (e.g. thickness uniformity of GaN/Si
problems had to be overcome. GaN/Si wafers suffer from √σ< 0.5 %). Theoretically, such processes could also be
a significant mechanical strain that is caused by the lattice carried out on 300 mm silicon wafers in the same way;
mismatch between substrate and epilayers, different the only limitation up to now, is the fact that the required
thermal expansion coefficients and thermal gradients 111-oriented silicon substrates are not available yet.
through the wafer.
In summary, switching to 4-inch and 6-inch GaN LED
This usually results in bowing, which is even more processes is the most immediately realistic opportunity to
pronounced the larger the wafer size. The bowing effect increase the productivity of HB-LED manufacturing. For
can lead to non-uniform wafer temperatures during the MOCVD as the key manufacturing step, the technical risk
growth process, which would inevitably result in non- and the cost related to the conversion to large wafers is
uniform film properties. very small. The availability of substrates and their cost are
likely to be the key parameters that determine the speed
Not all is lost though as there are ways to manage these of this conversion.
issues. Key to this is the use of in-situ monitoring of the
layer growth during the growth process, which then Employing silicon instead of sapphire is an interesting
allows the adjustment of the layer structure and the longer-term route to reduce manufacturing cost further, as
process to minimize strain and bow. complex processes on wafers up to 300 mm will become
commercially feasible. However, a significant improvement
The in-situ monitoring tool employed here was the in the efficacies (lm/W) of LEDs grown on silicon will be
ARGUS mapping system. In this device, a set of photo required to become competitive with established sapphire
diodes monitors the light emitted from the wafer surface based LEDs.
Employing silicon as a substrate instead of sapphire is an interesting
longer-term route to reduce manufacturing cost further, as complex
processes on wafers up to 300 mm will become commercially feasible
January/February 2010
www.compoundsemiconductor.net 33
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