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technology MOCVD
rhombus4
the MOCVD workhorses of the LED industry today, the
AIXTRON 2800G4 HT, one may first calculate the total Configuration 42x2” 11x4” 6x6”
wafer area that can be loaded into the reactor (for the Total wafer area
“standard” configuration of 42x2-inch wafers and the (cm
2
) 851 891 1094
respective 4-inch and 6-inch configurations). In a second
step, a certain amount of rim area (which is usually Total usable area
excluded from the usable area) is taken into account (cm
2
, 3 mm rim
additionally. Finally, the gained usable area is calculated excluded) 662 789 1009
compared to the initial 42x2-inch configuration.
Gain compared to
The potential result is impressive – 53% of the usable 42x2” 19.3% 52.6%
wafer area can be gained by simply moving to larger
6-inch wafers and most importantly – there is not a Table 1. The benefits of larger wafers
significantly high cost related to the conversion of a 2-inch
reactor to a 4-inch or 6-inch configuration. Although the there when customers decide to convert or buy 6-inch
prices for 4-inch substrates today are now in an wafer configurations.
acceptable commercial range, the prices of 6-inch wafers
are only just beginning to move downwards as the initial For those who do decide to convert; all that is required is
volumes begin to rise. a simple exchange of each of the six 7x2-inch satellite
disks in a 42x2-inch configuration in a AIX 2800G4 HT
Looking at these economics, it may appear surprising that over to 1x6-inch disks (Fig.1), converting the reactor to a
most of the GaN epitaxy is still performed on 2-inch 6x6-inch configuration.
sapphire wafers and indeed recently, many of the larger
GaN LED manufacturers have started to convert their Theoretically, as the reactor geometry has not been
volume production over to 4-inch substrates when buying changed, there is also a need to adjust or modify the
new systems for increased capacity. process. When the conversion is complete, the process
results are impressive. Uniformity levels are achieved that
Whereas we know that changing to 6-inch substrates lead to significantly improved yield (see Fig. 2). Looking
clearly will be the next step for the industry, the decision further ahead, 6-inch wafers might not be the final limit.
to initiate this transition will depend on sufficient substrate Some early 200 mm sapphire substrates are already
availability and commercially viable prices. However, when available today, and some initial studies into the MOCVD
that time arrives, the reactor technology needed is already growth on such wafers have already been carried out.
Fig.2: PL uniformity of a GaN/InGaN MQW grown in Fig. 3: Thickness uniformity map of a GaN film on 200
a 6x6-inch configuration. Standard deviation of mm sapphire substrate. Standard
wavelength is 1.2 nm deviation of thickness is 1.9%
32 www.compoundsemiconductor.net January/February 2010
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