Currently, in LED manufacturing processes, although sapphire substrates face challenges from Si and GaN substrates, considering cost and yield, sapphire still has an advantage in the next two years. It is foreseeable that the future development direction of sapphire substrates will be large size and patterned (PSS).
I. LED Sapphire Substrate Processing Technology
Firstly, for a sapphire substrate, it undergoes several processes before becoming a qualified substrate, including cutting, rough grinding, fine grinding, and polishing. Taking a 2-inch sapphire substrate as an example:
1. Cutting: Cutting involves using a wire EDM machine to cut sapphire crystal rods into rough sheets with a thickness of approximately 500µm. In this process, diamond wire saws are the most important consumable, currently mainly sourced from Japan, South Korea, and Taiwan.
2. Rough Polishing: The surface of the sapphire after cutting is very rough and needs to be rough polished to repair deeper scratches and improve the overall flatness. This step mainly uses 50~80um B4C with Coolant for grinding, and the surface roughness Ra after grinding is about 1um.
3. Fine Polishing: The next step is more refined processing, as it directly affects the yield and quality of the final product. Currently, the standard thickness of a 2-inch sapphire substrate is 430µm, so the total removal amount during fine polishing is approximately 30µm. Considering the removal rate and the final surface roughness Ra, this step mainly uses polycrystalline diamond liquid with a resin tin pad in a lapping process.
To ensure stability, most sapphire substrate manufacturers use Japanese grinding machines and original polycrystalline diamond slurry. However, with rising cost pressures and improved domestic consumable standards, domestic consumables can now replace original products and significantly reduce costs.
It's worth elaborating a bit more on polycrystalline diamond slurries. For the micronized portion of polycrystalline diamond slurries, a high particle size and regular morphology are generally required. This provides sustained cutting force and results in more uniform surface scratches. Domestic manufacturers capable of producing polycrystalline diamond micronized powder include Beijing Guoruisheng and Sichuan Jiuyuan. Guoruisheng also produces its own diamond slurry, giving it a significant advantage in both quality and cost. Diamond Innovation in the US recently launched "quasi-polycrystalline diamond," essentially an improvement on ordinary monocrystalline diamond. While its more robust structure provides higher cutting force, it also makes it more prone to deeper scratches.
4. Polishing: Although polycrystalline diamonds cause significantly fewer scratches than monocrystalline diamonds, they can still leave noticeable scratches on the sapphire surface. Therefore, a CMP polishing process is required to remove all scratches and leave a perfect surface. Originally a planarization process for silicon substrates, CMP is now equally applicable to sapphire substrates. After CMP polishing, the sapphire substrate undergoes multiple inspections, and only those that meet the standards are sent to the epitaxial wafer fabrication plant for epitaxy.
II. Backside thinning process of the chip
After epitaxy, the sapphire substrate becomes an epitaxial wafer. Following a series of complex semiconductor processes including etching, vapor deposition, electrode fabrication, and protective layer fabrication, the epitaxial wafer needs to be cut into individual chips. Depending on the chip size, a 2-inch epitaxial wafer can be cut into thousands to tens of thousands of chips. As mentioned earlier, the thickness of the epitaxial wafer at this stage is around 430µm. Due to the hardness and brittleness of sapphire, ordinary cutting processes are difficult to perform. Currently, the common process is to thin the epitaxial wafer from 430µm to around 100µm before using laser cutting.
1. Grinding process:
While lapping provides good processing quality for epitaxial wafers, its removal rate is too low, reaching a maximum of only about 3µm/min. If lapping is used throughout the entire process, this step alone would take about 2 hours, resulting in excessive time costs. The current solution is to incorporate a grinding process before lapping, using a diamond grinding wheel and a thinning machine to achieve rapid thinning.
2. Lapping process
After thinning, a polycrystalline diamond solution of approximately 6µm is used in conjunction with a resin copper disk. This achieves a high removal rate and repairs deeper scratches left by the grinding process. Generally, cracks during the cutting process occur because deeper scratches from the grinding process were not removed, thus requiring a high-quality diamond solution.
Besides chip cracking, some chip manufacturers plate copper on the back of the epitaxial wafer after the lapping process to increase chip brightness. This places higher demands on the surface finish after lapping. While some scratches may not cause chip cracking, they can affect the back plating effect. In such cases, a 3µm polycrystalline diamond solution or even a finer microparticle can be used for the lapping process to achieve better surface quality.
