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MOCVD tools  industry


Reactor design considerations Based on these conclusions, our development team has focused on reactor improvements that will reduce epitaxy cost in the short term. However, this was not our only goal. One of the great strengths of our Crius tools is their incredibly high level of stability – this is a major aid to process engineers, who don’t have to keep tweaking their recipes from one run to the next. We decided that it was unacceptable to compromise this stability and reproducibility, and if it could be improved, so much the better.


Finally, and possibly the most important condition of all, we decided that we had to be able to guarantee a seamless process transfer from the CRIUS II reactor to its successor. To meet all these requirements, we selected an evolutionary approach that resulted in the development of the enlarged CRIUS II-L reactor.


This MOCVD tool can accommodate many wafer sizes: 69 x 2-inch, 16 x 4-inch, 7 x 6-inch or 3 x 8-inch (see Figure 3). Switching between these sizes is simple – it just requires an exchange of the susceptor plate. No adjustments to the process are needed.


Scaling up the process for the Crius II-L reactor is just as straightforward as it was for migrating between previous Close Coupled Showerhead reactor generations. All the gas flows can be scaled up by the same amount because the major geometrical and mechanical parameters are unchanged and only the susceptor and showerhead diameter have increased. The area of the new susceptor is 9 percent bigger than its predecessor, so gas flow rates for carrier gases, ammonia and group III alkyls must be increased by that amount. However, thanks to gains in the susceptor fill factor, 25 percent more wafer area is utilized, translating into a reduction in metal organic and ammonia consumption by 13 percent, and ultimately a significant cut in LED manufacturing costs.


Crius reactors are renowned for excellent levels of uniformity that hold the key to impressive yields. This uniformity stems from the combination of homogeneous gas injection through the proprietary showerhead and a unique heating system design that provides extremely uniform temperature distribution. It is imperative that these foundations underpinning the success of the Crius feature in the II-L, so special care was taken to design a heater delivering the required level of temperature uniformity across the entire growth area.


Wavelength uniformity of epiwafers produced on the II-L showcases the success of these efforts. In runs using a full load of 2-inch wafers, the typical wavelength uniformity for epiwafers with a peak emission of 460 nm


Fig.2: Total cost of ownership breakdown for a GaN LED MOCVD system.Reactor related costs account for more than 90 percent of the total cost


Fig.1: Cost reduction roadmap for packaged LEDs (Courtesy of the United States Department Of Energy’s Office of Energy Efficiency and Renewable Energy)


was just 0.9 nm, in terms of the standard deviation (2 mm edge excluded). Variations in wavelength between all 69 wafers can be as low as 3.1 nm.


Similarly impressive results are obtained for a full 16 x 4- inch reactor load: 1.3 nm uniformity on each wafer and an absolute wavelength range of just 2.1 nm for all 16 wafers (see Figure 4). Values from epitaxial yield can be calculated from the uniformity figures, and indicate that


October 2011 www.compoundsemiconductor.net 45


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