Eliminate the internal stress of sapphire crystal material in one step

Stress is inevitable in the growth of sapphire crystals by the temperature gradient method (TGT). In order to improve the quality and processing quality of the crystal, it is necessary to carry out stress relief decoloring annealing treatment. Using the existing "two-step" annealing process, due to its low annealing temperature and slow speed, it needs to pay a higher time cost and the cost of the annealing process. Now a "one-step" annealing process is proposed, that is, direct high-temperature annealing in an air atmosphere of 1750 to 1800 ℃. After the sapphire crystal bar is annealed by the "one-step" method, its interior is clear and transparent, and the warp and bow defect rates of the final wafer processing inspection are reduced by 100% and 70%, respectively, compared with those before the annealing . The high temperature annealing treatment effect of the sapphire crystal rod is obvious, and it can be tried and promoted in other crystal materials.

In the past few decades, sapphire crystals have been widely used, such as the most practical substrate material for GaN blue-green light semiconductor diodes, ideal window materials for high-power lasers, and laser matrix materials. At present, various grades of sapphire crystals, such as 80 kg grade, 120 kg grade and 150 kg grade, can be produced by the temperature gradient method and the Kyropoulos method.

In the process of sapphire crystal growth, stress will inevitably appear in the crystal. Especially for sapphire crystal rods, additional processing stress will be introduced after the rod processing process. The sapphire crystal rod with stress may cause problems such as large arc of the wire bow and low cutting accuracy when cutting. These problems may increase the stress on the cut surface of the wafer, and make the warp and bow of the wafer after the cut larger. In addition, after the wafer is conventionally annealed (around 1400 ℃), the internal stress of the wafer cannot be released. Therefore, the wafer is prone to chipping, cracking, warping, astigmatism, etc. during subsequent grinding and chamfering. Will further reduce the yield of sapphire wafers. Therefore, in order to improve the yield rate of wafer processing, it is very necessary to eliminate the internal stress of the sapphire ingot in time. In addition, in a growth furnace using a tungsten cage as a heating element, tungsten atoms (W) and a small amount of molybdenum atoms (Mo) may randomly enter the melt in the form of solid diffusion and infiltration at high temperatures, causing lattice distortion in the crystal. Defects such as, dislocations, and grain boundaries may also affect the quality of sapphire crystals.

According to the existing reports, Xu Jun et al. used the sapphire crystals grown by the temperature gradient method (TGT method) to hold the sapphire crystals in a medium-high temperature oxidizing atmosphere (1600 ℃ air atmosphere) for 120 h, and a high temperature reducing atmosphere (1900 ℃ hydrogen atmosphere) for 120 h. Stress-relieving and decolorizing annealing treatment, that is, "two-step" annealing treatment, the crystal becomes colorless and transparent [1]. After testing, the integrity and optical transmittance of the sapphire crystal have been significantly improved. However, the two-step method is used for annealing treatment, and the time cost and annealing process cost are relatively high. After preliminary process exploration and research, we now propose a "one-step" high-temperature annealing process, that is, direct high-temperature annealing in an air atmosphere at 1750 ~ 1800 ℃, and the holding time depends on the crystal quality (generally no more than 40 h). First, the low-valence impurity ions (such as Cr3 +, Ti3 +) that may be contained in the sapphire raw material are fully oxidized to the high-valence state (Cr4 +, Ti4 +) to further reduce the color difference of the ingot and increase the quality of the ingot. Second, tungsten and molybdenum atoms (W & Mo) are oxidized to produce WO3 and MoO3, which are easy to sublime at high temperatures, and W & Mo atoms leave the ingot in a gaseous form. Third, high-temperature annealing is the best method to eliminate the internal stress of the crystal, and the higher the temperature, the shorter the annealing time [2-3], and compared with the existing two-step process, this process greatly shortens the annealing cycle.

The outline diagram of the high temperature annealing furnace (LGYL-1800AAA-30, Nanjing, Polytechnic Yulong New Materials Polytron Technologies Co., Ltd., China) used in this process is shown in Figure 1. Put the to-be-annealed sapphire crystal rod (or other sapphire product) in an annealing furnace to anneal, increase to 1750 ~ 1800 ℃ according to the established heating curve and keep the temperature constant for about 40 hours, and then reduce to room temperature according to the established cooling curve. The entire process obviously shortens the annealing cycle of the sapphire ingot, and saves the time cost and process cost of annealing.