LEDmanufacturing industry
T
he LED business is booming. These chips are generating attractive cash flows from backlighting
the screens of netbooks, lap tops and TVs and this solid- state device is about to break into lucrative new territory: general illumination. The leading LED manufacturers have had their eyes firmly fixed on this goal for many years and their dream is now turning into reality, thanks to the release of the first commercial lighting products.
At Aixtron, which is based in Aachen, Germany, we have a strong track record in supporting the tremendous progress of LED manufacturers. Our effort has focused on continuous improvement in the throughput of MOCVD reactors, echoing the developments of other toolmakers in the silicon industry.
Our first design of MOCVD reactor for growing GaN- based LED epistructures accommodated 2-inch substrates, and over the years we have unveiled reactors that can house more wafers with larger diameters. This effort has culminated in our release of the future-proof Aixtron AIX G5 HT earlier this year. This tool offers simultaneous deposition of GaN and its related alloys on eight 6-inch wafers, the size that many LED chipmakers will look to migrate to over the next few years. In addition, this reactor can be configured for the growth of multiple 8-inch wafers.
The economies of scale realized by changing to a 6-inch process are obvious: better utilization of the MOCVD reactor area; less edge exclusion; more efficient handling; and better precursor utilization in the epitaxial process. However, it is not possible to produce high-quality, 6-inch LED epiwafers by simply taking established processes and applying them to these larger wafers. That’s because such large wafers create their own challenges due to their size, weight, and thickness, and the entire MOCVD environment has to be designed to suit them. In addition, the MOCVD tool must be capable of high yields and fast cycle times, as otherwise this would negate the productivity advantage gained by the migration to large wafers.
We considered these issues when we defined our requirements for our 6-inch MOCVD tool. We decided that the reactor must be capable of producing epiwafers with uniformity high enough to translate to an overall gain in yield over previous generations of MOCVD tools. For the same reason, we had to build a reactor that set a new benchmark for wafer-to-wafer, run-to-run and tool-to-tool reproducibility.
To maximize throughput, our reactor would have to operate without cleaning and baking between growth runs. In addition, we set out to build a tool that required very little preventative maintenance, generated very few particles, and was highly automated. For example, customers had to have the option of buying a version of this tool with automatic loading and unloading.
Our flagship reactor, the AIX G5 HT, fulfils all these goals by realizing stable, reproducible and uniform growth processes on wafers up to 8-inch in diameter (see Figure 1). While designing this reactor, we paid careful consideration to the two fundamental aspects that determine the capabilities and performance of any MOCVD reactor: the thermal conditions; and the gas flow dynamics and chemical reactions, in both the gas phase and the solid phase.
Minimizing temperature variations The AIX G5 HT features a novel type of gas injector that introduces perfectly laminar gas flows into the reactor. This condition can be realized at high growth pressures (close to atmospheric pressure) and growth rates of up to 30 µm/hr. Thanks to this approach, uniform gas phase depletion occurs for all wafer sizes.
One pre-requisite for the growth of high-quality epiwafers is excellent temperature uniformity across the wafer — deviations must be less than 1 °C. This must be realized for both the low temperatures associated with multi- quantum well (MQW) growth, and the far higher temperatures employed for growth of the other regions of the LED.
To realize the excellent temperature uniformity that holds the key to uniform film deposition, we employ our proprietary Planetary Reactor design that features on all of our multi-wafer tools. The satellite disks that hold the wafers rotate individually on a rotating planet disk, which is heated by an RF coil. For large wafer sizes such as 6-inch, this principle leads to an inherent advantage for the subsequent backend process. This stems from the high degree of rotational symmetry associated with the temperature distribution on the satellite, and the wafer that it supports.
To improve reactor performance even further, we have optimized the design of the RF coil and the satellite disk. Thanks to this, our tool delivers unprecedented levels of uniformity on 6-inch wafers.
November / December 2010
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