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              HOME  >  LED knowledge  >  LED epitaxial chips  >  LED sapphire substrate and chip back thinning process analysis
              LED epitaxial chips

              LED sapphire substrate and chip back thinning process analysis

              Date:2018-08-10

              In the gradual exploration, the industry has formed a set of roughly the same process for thinning and flattening sapphire substrates.
              At present, in the LED process, although the sapphire substrate is challenged by Si and GaN substrates, sapphire has certain advantages in recent years in consideration of cost and yield, so it is very difficult to reduce and flatten the surface. In the gradual exploration, the industry has formed a set of roughly the same process for thinning and flattening sapphire substrates.


              First, LED sapphire substrate processing technology

              First, for a sapphire substrate, it undergoes several steps from cutting, rough grinding, fine grinding, and polishing before it becomes a good substrate. Take 2 inches of sapphire as an example:

              1. Cutting: Cutting is performed from a sapphire ingot through a wire cutter to a thickness of about 500 um. In this process, diamond wire saws are the most important consumables, currently mainly from Japan, South Korea and Taiwan.

              2. Rough throw: The surface of the sapphire after cutting is very rough and needs to be rough thrown to repair the deeper scratch and improve the overall flatness. This step is mainly carried out by using 50~80um B4C plus Coolant, and the surface roughness Ra after grinding is about 1um.

              3. Fine polishing: The next step is finer processing, because it is directly related to the yield and quality of the final product. The current standardized 2-inch sapphire substrate has a thickness of 430 um, so the total removal of fine polishing is about 30 um. Considering the removal rate and the final surface roughness Ra, this step is mainly processed by Lapping in a polycrystalline diamond liquid in combination with a resin tin plate.

              In order to achieve stability, most sapphire substrate manufacturers use Japanese grinding machines and original polycrystalline diamond liquids. However, with the increase of cost pressure and the improvement of domestic consumables, domestic consumable products can replace the original products and significantly reduce costs.

              Speaking of polycrystalline diamond liquid may wish to say a few more words, for the micro-powder part of polycrystalline diamond liquid, it is generally required to concentrate the granularity, the shape should be regular, which can provide long-lasting cutting force and surface scratching is relatively uniform. Domestic manufacturers that can produce polycrystalline diamond micropowders include Beijing Guoruisheng and Sichuan Jiuyuan, while Guoruisheng can also produce diamond liquid by itself, so it has great advantages in quality and cost. DiamondInnovation in the United States recently introduced "polycrystalline diamonds", which is actually an improvement on ordinary single crystal diamonds. Although the relatively strong structure can provide high cutting force, it is also more likely to cause deeper scratches.

              4. Polishing: Although the scratch caused by polycrystalline diamond is obviously smaller than that of single crystal diamond, it will still leave obvious scratch on the surface of sapphire. Therefore, it will be polished by a CMP to remove all scratches and leave a perfect surface. . The CMP process was originally a process for planarizing a germanium substrate, and is now equally applicable to sapphire substrates. The sapphire substrate subjected to the CMP polishing process is subjected to layer inspection, and the qualified product can be delivered to the epitaxial factory for epitaxy.

              Second, the chip back thinning process

              After the epitaxial sapphire substrate becomes an epitaxial wafer, the epitaxial wafer needs to be cut into a single chip after a series of complicated semiconductor processes such as etching, evaporation, electrode fabrication, and protective layer fabrication, according to the size of the chip. A 2-inch epitaxial wafer can be cut into thousands to tens of thousands of CHIPs. As mentioned above, the thickness of the epitaxial wafer is around 430 um. Due to the hardness and brittleness of sapphire, it is difficult to process it by ordinary cutting processes. The current common process is to thin the epitaxial wafer from 430um to around 100um, and then use laser to cut.

              Grinding process:

              Although the processing quality is good for the epitaxial film in Lapping mode, the removal rate is too low, and the highest can only reach about 3um/min. If Lapping is used in the whole process, only the processing takes about 2 hours, and the time cost is too high. . The current solution is to add Grinding's process before Lapping, and achieve the purpose of rapid thinning through the cooperation of diamond grinding wheel and thinning machine.

              Lapping process

              After thinning, the use of 6um of polycrystalline diamond liquid combined with resin copper plate can achieve higher removal rate and repair the deeper scratch left by the Grinding process. Generally, the rupture occurs during the cutting process because the deep scratches in the Grinding process are not removed, so the requirements for the diamond liquid are relatively high.

              In addition to the lobes, some chip manufacturers in order to increase the brightness of the chip, after the Lapping process will also be copper plated on the back side of the epitaxial wafer, at this time put forward higher requirements on the surface after Lapping. Although some scratches do not cause cracks, they can affect the effect of back plating. At this point, a 3 um polycrystalline diamond solution or a smaller nuance can be used for the Lapping process to achieve better surface quality.

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