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substrates  industry


Economies of scale


Many leading LED chip manufacturers have already moved from 2-inch to 4-inch substrates, and they are now announcing their intentions to migrate to 6-inch. The benefit of this move – and what is possible by moving to even larger sizes – is illustrated in Table 1.


One benefit of turning to bigger wafers is that more LED chips can fit along the outer perimeter due to reduced curvature, especially for larger high-brightness chips. This so-called ‘edge-effect’ provides incrementally more chips than just a raw calculation of geometrical area. For example, the surface area of a 6-inch wafer is nine times that of a 2-inch wafer, but it can yield between ten and twelve times as many chips.


Switching to growth on larger substrates also leads to more efficient film deposition. That’s because large diameter wafers provides more efficient utilization of useable surface area in the MOCVD reactor, thereby increasing chip productivity. According to a recent study by the German tool-maker Aixtron, if a platter in an MOCVD tool is filled with seven 6-inch substrates rather than 42 substrates that are 2-inch in diameter, throughput increases by 52 percent. These significant gains in cost reduction mirror the savings made in the silicon industry as it has scaled production to larger and larger wafers.


An additional incentive for moving to larger wafers is the existing legacy equipment and expertise with large wafers. Many LED companies have a heritage in the optoelectronics or semiconductor industries, and are already possess large diameter equipment and process expertise for 6- and 8-inch LED production. What’s more, they understand the logistical issues involved in migration to large diameter wafers.


Meeting the needs From the perspective of a sapphire substrate maker, increasing wafer size is not trivial. LED makers need substrates that are flat, uniformly polished, and have a surface free from contamination. Meeting these tight product specifications becomes increasingly difficult with larger substrate sizes.


Another key requirement made by LED manufacturers is for the sapphire substrates to be stress-free. During the MOCVD process these wafers are subjected to temperature cycles of up to 1200° C, which can introduce stress and ultimately lead to small cracks.


At Rubicon, headquartered in Bensenville, IL, we have developed proprietary crystallization and wafer processing technologies to produce stress-free crystals in an unconstrained environment most mimicking nature. We use this unconstrained process to manufacture 6-inch wafers that not only meet industry requirements, but exceed them. What’s more, our


wafers offer an excellent surface for MOCVD growth, thanks to our expertise in both equipment customisation and polishing process development. Such efforts are needed, because slicing, lapping and polishing this material is not easy – sapphire is the second hardest naturally occurring material on earth after diamond, and cutting and polishing it requires specialized equipment and process expertise.


We are able to produce our substrates in high volume, and the variation between them is minimal. This is a major asset for LED makers, who can only turn in a profit if they can churn out wafers with very small variations in properties such as emission wavelength and brightness. Delivering on these fronts means that the wafer yields a high proportion of LEDs that meet their key specs, such as those for colour and efficacy.


How large can we go? The key to producing high-quality, large diameter wafers is an end-to-end stable process that starts with crystal growth and goes right through to wafer polishing. It is impossible to make such wafers without a high-quality, single sapphire crystal boule. We, along with some of our competitors, are well placed in this regard because we are vertically integrated. This means that we can ensure quality from the raw material, aluminium oxide,


According to analysis by Yole Developpé- ment,6-inch sapphire will provide the foundation for the majority of LED chips by the middle of this decade


Thanks to edge effects,a four-fold increase in wafer diameter that results from a doubling of substrate diameter increases chip production quantities by up to a factor of five.Numbers are estimated,based on typical dimensions for a high-brightness chip


October 2011 www.compoundsemiconductor.net 41


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