Polishing process is the key technology in sapphire wafer processing

Source: Journal of Intraocular Crystals
At present, the mainstream of sapphire polishing industry still uses nano silica as polishing liquid, and finally the roughness of polished surface (Ra, atomic force microscope test) is below 0.3nm. In the polishing process, the reaction of the silica polishing liquid with the sapphire surface is shown in the following formula, and the product is the dihydrate of aluminum silicate. Al2O3 + 2 sio2 + 2 h2o = Al2Si2O7 · 2 h2o
Although aluminum oxide abrasive can be used as sapphire polishing liquid, but because now the production of aluminum oxide abrasive is generally through calcination, grinding, screening, to get uniform consistency, particle size of nanometer aluminum oxide is very difficult. The most deadly is aluminum oxide hardness, polishing surface scratches seriously; Large viscosity, polishing surface is not easy to clean, it is difficult to ensure the surface smoothness of polishing materials. Although it has been reported that aluminum oxide is used as polishing fluid, it is only in the experimental stage at present. The main problems are: (a) high speed requirement for equipment; (b) Sapphire surface roughness is relatively large; (c) slightly more surface defects; (d) Higher costs. AbhayaK. Bakshi et al. from the United States used α or β silicon carbide polished sapphire with a particle size of 100 ~ 400nm. They softened the silicon carbide surface with a silicon dioxide coating in the hope of reducing scratches on the sapphire. The reason is that the silicon dioxide coating can passivate or reduce the sharp corners of silicon carbide particles. The core of the particle is still silicon carbide with greater hardness, and the abrasive maintains a better polishing removal rate. They also use silicon carbide and silicon dioxide composite polishing liquid (30%SiC, 70%SiO2) to process R direction sapphire, compared with the silicon dioxide polishing liquid phase without adding silicon carbide, the polishing rate can be increased by 30% ~ 50%.
Adding catalyst (Fe-Nx/C) to silica polishing liquid can improve the polishing rate of sapphire by about 15%. The catalyst contains ferric oxide, ferric oxide, pyridine type nitrogen, pyrrole type nitrogen. Ultrasonic vibration can significantly improve the removal rate of sapphire polishing process with SiO2 silica sol (about 3.8 times) and reduce the roughness. Ultrasonic waves are added to the toss of a single - sided polisher. The possible explanations are as follows: (1) the movement path of silica particles on sapphire surface is lengthened; (2) The contact pressure between silica particles and sapphire is increased; (3) Ultrasonic vibration and gas corrosion strengthen the interaction between silica particles and sapphire surface.
At present, the crystal direction of sapphire material is generally C direction. Due to the high hardness and poor chemical activity of sapphire material, its polishing rate is relatively slow compared with that of other materials. At present, the polishing rate of sapphire in the world is not more than 5 ~ 10μm/h. In the laboratory, Zong Simiao and Liu Yuling of Hebei University of Technology used self-developed polishing liquid at a high pH value of about 12.5, a high abrasive concentration (50%) and a rapid polishing temperature up to 45℃, and the polishing rate of sapphire can reach 11.3μm/h. Li Shurong and Jin Zhuji from Dalian University of Technology studied the effect of different pH regulators on polishing rate, compared four inorganic bases including sodium hydroxide, potassium hydroxide, calcium hydroxide and ammonia, and found that potassium hydroxide can significantly improve the polishing rate of sapphire. Yu Jiangyong and Liu Yuling of Hebei University of Technology added nano-scale alumina particles (concentration of 2%) to SiO2 polishing liquid, which can improve the polishing rate of sapphire from 9μm/h to 11.3μm/h, and the dilution rate of silica polishing liquid increased from 1∶1 to 1∶2. In addition to the polishing equipment, the main factor affecting the polishing rate is the polishing fluid. In addition, the A-direction, M-direction and R-direction polishing rate is slower than that of C-direction sapphire. The possible reason is that the direction of C towards sapphire crystal is (0001), and the atomic bond is O-Al-Al-O-Al-Al-O; The M and A directions in the sapphire crystal are (10-10) and (11-20), respectively, and their chemical bond is Al-O-Al-O. Because Al-Al bond energy is smaller and bonding is weaker than Al-O bond, C-sapphire polishing is mainly to open Al-Al bond, so the polishing rate is relatively fast. It is particularly urgent to develop multi series of high efficiency sapphire polishing liquid.
Polishing process is divided into single - sided polishing process and double - sided polishing process. In general, the sapphire wafer processed by double-sided polishing equipment is better than the sapphire wafer processed by single-sided polishing equipment, which is mainly determined by the principle of double-sided polishing. In the direct double-sided polishing process, the wafer can achieve rotation and a wide range of revolution. In this way, the flatness of the polished film is better. In addition, in the process of direct double-sided polishing, the force on the front and the back is equal, synchronous polishing, so that the bending degree of sapphire sheet is better.
As the quality requirements of final sapphire products become higher and higher, the requirements of surface quality also increase. The quality of sapphire surface depends on the final wet chemical cleaning, in addition to the last step of buffingpolish before the end of polishing is also very important, often ignored. The characteristics of light pressure polishing are light pressure, fast speed and short time. The finish of light pressure polishing can reduce the surface roughness and surface defects.