German intellectual property licensing company ALLOS Semiconductors positions itself as "a leading supplier of silicon-on-silicon (GaN-on-Si) wafer technology for LED, power and RF applications" and claims to have the best on the market Crystal quality, uniformity, and low wafer warpage.
ALLOS CEO Burkhard Slischka talks about the company's proprietary technology and his vision for future Micro LEDs.
Q: Why is silicon-based gallium nitride? Almost all traditional LEDs are produced using sapphire substrates. Why are Micro LEDs different?
A: In order to successfully produce Micro LEDs, fundamental changes are necessary, for example, with new equipment design and novel manufacturing techniques. In addition, the most important thing is that the production yield must be much higher than the current LED industry. For all of these changes, silicon GaN is a key driver and can save costs.
Q: In your opinion, silicon GaN has a better performance in terms of production yield and cost.
A: Yes, that's it. To understand this, we need to look at the entire value chain from the epitaxial wafer growth to the full process of the Micro LED chip being successfully applied to the display.
Q: In this value chain, what do you think is the biggest impact of using a silicon-based gallium nitride substrate?
A: There are three factors on the entire chain, namely the cost advantage of larger wafer diameters, the advantages of silicon film processing, and the large impact of wafer uniformity on yield.
Q: At the end of 2017, Veeco and ALLOS jointly announced that ALLOS technology can achieve the best uniformity on 200 mm silicon GaN wafers. How is GaN compared to sapphire substrate?
A: We show that the wafer wavelength uniformity is less than 1 nanometer, and the thickness uniformity is even better. The 200 mm silicon gallium nitride wafer and the 150 mm sapphire substrate GaN wafer perform equally well in these respects. In addition, our technology has very good wafer reproducibility.
The next goal is to achieve 99% wafer area in the ±1 nm bin range
Q: Do you think there is room for improvement in the future?
A: Yes, of course we can. In just four weeks, we achieved these results through Veeco's Propel reaction chamber. So there is still a lot of room for improvement. The next goal is to achieve 99% wafer area in the range of ±1 nm bin.
Q: Compared to the uniformity achieved by others on a 150 mm sapphire substrate, do you think ALLOS can achieve the same or even better uniformity on 200 mm GaN?
A: Yes, the uniformity is the same, but the wafer area is almost twice.
Q: Excellent uniformity seems to be much more important for Micro LEDs than for traditional LEDs. Why?
A: Uniformity is a key driver of yield. Each display requires millions of Micro LED chips with identical wavelengths. Classification of the LED chips prior to assembly or any subsequent pixel repairs will increase the cost of the display. Therefore, this needs to be solved with a very uniform epitaxial material from the beginning, otherwise the Micro LED display will not be cost competitive.
Q: In this case, which one do you think is the best wafer uniformity and the lowest epitaxial cost per wafer area?
A: This is a very good question. It is unscientific to look at one of these two alone. Because wafers with better uniformity can increase the yield of all subsequent manufacturing steps. Therefore, even if the extension cost is higher, the cost gap can be compensated for by better yield.
"Save 45% with 200 mm CMOS"
Q: This led to the second factor we mentioned earlier, the cost advantage of larger wafer diameters. As far as I know, turning to a larger wafer diameter will reduce the cost per chip. But would it be the same for 200 mm sapphire-based GaN?
A: Yes, in principle, you should think so. In addition, there are three problems with sapphire:
1. The high cost of a 200 mm sapphire substrate;
2. The required wafer uniformity may not be achieved;
3. Due to the large degree of warpage, it is possible to obtain sufficient yield.
But more important is another advantage of silicon-based gallium nitride: the existing 200 mm complementary metal-oxide-semiconductor (CMOS) processing capability can be used, and the cost savings per region is expected to be 45%.
At the same time, it provides the high process yields required for Micro LEDs, which can only be matched after large capital expenditures and skill upgrades.
Q: Is the 45% you mentioned only from chip processing?
A: Yes, the higher benefits come from substrate removal and bonding, as well as a better “area economy†for massive transfers.
Q: There is another technical point that I want to know. You just said that it is difficult to achieve uniformity on sapphire, how to achieve uniformity on 200 mm silicon?
A: Our silicon-based GaN technology is equipped with unique strain engineering technology. This allows us to achieve two things at the same time: during the growth process, we can achieve very good temperature uniformity; after cooling, a flat, crack-free wafer is formed. Usually the two are compromised, but we can provide both at the same time. By the way, we are very confident to adjust this ratio to 300 mm.
“A flat and crack-free 300 mm is possibleâ€
Q: 300 mm silicon gallium nitride?
A: Yes, 300 mm is possible. Unfortunately, there is currently no 300 mm industrial grade MOCVD equipment. But we are optimistic that it will change. We will soon be able to showcase the first flat, crack-free 300 mm GaN-on-Si epitaxial wafer.
Q: It's fun! I understand the cost advantage of the GaN-on-Si that you just introduced for Micro LEDs. But what is the difference in performance from GaN-on-sapphire? We understand that there is concern in the industry about the brightness and light efficiency of GaN-on-Si.
A: In fact, using silicon-on-GaN (GaN-on-Si) to achieve good enough crystal quality is very difficult, which can damage the performance of the LED you mentioned. We have overcome this shortcoming and achieved the best crystal quality in the industry using silicon-based gallium nitride. In fact, our top layer defect density is only 2 x 108 cm-? TDD, which is equivalent to the value of the sapphire base.
Q: What does it mean for the performance of LEDs?
A: This means that once we authorize and transfer our technology to our customers, their existing luminescent layer grown on sapphire and our gallium nitride buffer on a silicon substrate will be combined into one formulation. This produces the GaN-on-Si epitaxial wafers we have all the best, and the same luminescent structure and performance that our customers now grow on sapphire.
"The same performance as sapphire base"
Q: We continue to discuss the business side. What is the impact of Micro LEDs on the supply chain?
A: The target application of the application plant will determine development and business decisions. Smartphones, TVs, or other types of device manufacturers either build Apple Micro's capabilities within Apple's model or motivate strategic partners to provide solutions for parts in the value chain.
But in either case, they need to coordinate the technology, benefits and costs of the value chain and, to some extent, fund research and development. It is not a viable strategy for me to wait for a vendor to provide an end-to-end solution for Micro LEDs.
Q: How does this affect the future business strategy of ALLOS?
A: ALLOS has powerful silicon-on-GaN (GaN) technology in three markets (LED, power and RF). We see that these three markets require different technologies and business structures, and we also need to apply different strategic and strategic partnerships in these three markets.
Q: What are the company's goals for Micro LED in the next two years?
A: In terms of Micro LED, we are working with large companies and startups to further develop the technology. Based on the outstanding performance we have achieved today and our strong intellectual property portfolio, we see that we are in a good position. It is equally important to have other suitable partners in the value chain and the funds needed to develop Micro LEDs and enter mass production.
Q: If I am an application device manufacturer that you just mentioned and will lead and sponsor such development, I will ask for the exclusive right to this research and development technology...
A: Yes, we have heard a lot of these sayings now. The correct cooperation may be a win-win solution.
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