Should high-brightness LED manufacturing pay more attention to process control now? If the answer is yes, what should we learn from traditional silicon-based integrated circuit manufacturing?
The answer to the first question is clear: just weigh the benefits of process control and the equipment and labor costs that need to be paid. Benefits of process control include improved yield and reliability, shorter production cycles, and faster time-to-market for new products. If the cost of process control is taken into account, these benefits will translate into better profitability. It can be seen that it is meaningful to strengthen the emphasis on process control.
Let's start with the defect rate of the LED substrate and epitaxial layer. The most advanced LED devices use a sapphire (Al2O3) substrate; on the upper surface of the polished sapphire substrate, the epitaxial layer of gallium nitride (GaN) is grown by metal organic chemical vapor deposition (MOCVD).
Epitaxy is the technique of growing another thin film of crystalline material on top of a crystalline material so that the crystal lattices will match each other, at least very similarly. If the lattice constant of the epitaxial film is different from the underlying material, this mismatch creates stress in the film. There is a large amount of lattice mismatch (13.8%) between gallium nitride and sapphire. Therefore, the gallium nitride "epitaxial layer" is a high stress film. The stress of the epitaxial film can increase the mobility of electrons/holes, thereby improving the device performance; on the other hand, the film under stress tends to have a large number of defects.
Common defects after epitaxial layer deposition include crypts, cracks, hexagonal bumps, crescents, circles, showerhead droplets, and localized surface roughness. Pits often occur during the MOCVD process and are related to temperature gradients due to warping of the wafer from the center to the edge. Large pits can cause a short circuit in the PN junction, causing device failure. Submicron pits are even more invisible, allowing the device to initially pass electrical testing, but can cause reliability problems after the device ages. Reliability issues often occur in real-world applications, causing greater losses than the yield problems typically detected during factory testing. Another drawback is the cracking caused by the film stress, which will also lead to serious losses in practical applications.
High-end LED manufacturers typically detect epitaxial wafers and record all defects that are more than about 0.5 mm in size. A virtual device unit is superimposed on the wafer, and any virtual unit containing serious defects will be screened out. If there are pits in these units, they will fail. If there are cracks, they will face higher risk reliability problems. In many cases, almost all edge units are scrapped. Especially for high-end LEDs used in automotive or solid-state lighting, defects are never allowed, which means that the reliability of such devices must be very high.
However, the defects found in the post-epitaxial detection are not all due to the MOCVD process. Sometimes the problem is due to the sapphire substrate. If the LED manufacturer wants to improve yield or reliability, it is important to understand the source of the problem.
The sapphire substrate itself may contain a variety of defect types, including sapphire crystal pits that are exposed during cutting and polishing; scratches caused by surface polishing; residues left by polishing paste or cleaning process; and can be removed or removed by cleaning particle. When these defects are present on the substrate, they may be enlarged during the epitaxial growth of gallium nitride, causing defects in the epitaxial layer and ultimately affecting the yield or reliability of the device.
The patterned sapphire substrate (PSS) is a substrate designed to improve luminous efficiency in high-brightness LED devices because it uses a standard lithography and etching process prior to epitaxy to form a regular array of bulwarks on the substrate surface. Although the use of the PSS method can reduce dislocation defects, the bridging between missing bulges or bulges can become hexagonal and crescent-shaped defects after deposition of the gallium nitride layer, which are generally a fatal threat to yield.
In order to improve yield and reliability, LED manufacturers need to accurately specify the maximum defect rate of the substrate by type and size—provided that the substrate can be manufactured to those specifications without over-selling the price to offset the benefits of improved yield. . LED manufacturers can also benefit from routine incoming quality control (IQC) defect inspection to ensure that the substrate meets its specifications—including the type and size of defects.
When the substrate size changes, for example, when converting from a 4-inch LED substrate to a 6-inch LED substrate, the substrate defect rate should be thoroughly tested. Historically, even in the silicon industry, when substrate manufacturers face mechanical, thermal, and other process challenges from larger, heavier crystals, the increase in crystal defects due to larger substrate sizes is also The initial troubles.
A further consideration for effective defect control during LED substrate and epitaxial layer fabrication is the classification of defects. Knowing whether a defect is a pit or a particle is more helpful than just knowing the number of defects. (Scratches, cracks, and debris on the substrate are more easily identified based on their spatial characteristics.) Advanced defect detection systems, such as KLA-Tencor's Candela product, are designed to contain multiple angles of incidence (vertical, tilt) and more Detection channels (mirror, "topography", phase) help to automatically classify defects into various types.
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