industry LEDmanufacturing
excellent photoluminescence uniformities using the 8 x 6- inch configuration of the G5 reactor.
This high level of uniformity leads to great yield figures. Based on the above uniformity data, an exact calculation of the area yield shows that more than 98 percent of the wafer area is in a 5 nm bin.
A worthwhile analysis of yield must not be restricted to a single wafer – it must consider wafer-to-wafer uniformity and reproducibility. To deliver on both these fronts, we have devoted substantial effort to optimizing the design of the reactor and the materials that it is built from. On top of this, we have made further gains by controlling the temperature of each individual wafer.
Figure 1. Aixtron G5 HT Planetary Reactor in 56 x 2-inch configuration (above). Top view of this reactor in 8 x 6-inch configuration (right). The tool can also be configured for 4-inch and 8-inch wafers
Unfortunately, complete absence of temperature variations on the satellite disk is no guarantee of highly uniform film deposition. That’s because there are differences in the lattice constants and thermal expansion coefficients of the sapphire substrate and the nitride-based LED heterostructure. Strain that results affects all wafers, although the bowing that it causes gets more pronounced as wafer size increases.
Needless to say, bowing is an impediment to uniform LED properties. To prevent this, it is possible to deposit the epiwafers on thick sapphire substrates (above 1 mm), employ in situ curvature measurements to monitor and correct for bow, and last but not least, insert special layer stacks into the LED heterostructure that minimizes bow. Armed with these techniques, it is possible to realize
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www.compoundsemiconductor.net November / December 2010
To realize individual temperature control, the temperature from the top of each satellite is measured with a pyrometric device. The gas flow of the gas-foil rotation drive of each satellite is then adjusted accordingly, bringing the temperature of the wafer back to its desired value.
Keep on running
Reproducibility is a key issue in high volume manufacturing environments employing many identical MOCVD tools running standardized growth recipes. If high yields are to be realized day-in, day-out, then every reactor must deliver exactly the same performance and results from one run to the next without any re-calibration.
We have analyzed the root causes of non-stability in various MOCVD systems and determined that they are predominantly related to small temperature drifts in the reactor set-up. Consequently, with our G5 reactor we have strived for a design with inherent temperature stability. One of the key features of this particular reactor is its novel graphite ceiling plate. In the Planetary Reactor designs, the ceiling plate defines the upper thermal boundary of the reactor. Even though it is not actively heated, it does influence the reactor’s thermal management.
The great strength of the new graphite ceiling is that its emissivity is unaffected by the deposition of materials onto its surface. This means that the thermal properties of the reactor are fixed, rather than depending of the number of growth runs already performed. This results in unprecedented reproducibility of all LED properties, from run to run and between different reactors.
Another route to increasing productivity of an MOCVD system is to reduce its cycle times. The G5 reactor excels in this regard. Not only does it enable very high growth rates that cut material deposition times — it also has very
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