The polishing technique of sapphire can be divided into Single Side Polishing (SSP) and Double Side polishing (DSP). Regarding the flatness (TTV) and warp (WARP) of sapphire, in general, the sapphire window substrates produced using double-side polishing equipment are better than the sapphire wafer produced using single-sided equipment, which is attributed to the principle of double-sided polishing. In the double-side polishing procedure, the sapphire windows go under both self-rotation and large-diameter revolutions, so that higher flatness could be obtained. Furthermore, the front and back sides are subject to equal stress and synchronous polishing progress, Therefore, the curvature of the sapphire wafer is also better.
The composition of the polishing solution is another critical element of great influence. At present, the mainstream of sapphire polishing is using nanoscale Silicon Dioxide (SiO2) as the polishing solution, the achieved roughness of the final polished surface could be below Ra 0.3nm in general. In the polishing process, the reaction between the SiO2 polishing solution and sapphire is shown in the following formula: Al2O3+2SiO2+2H2O=Al2Si2O7·2H2O. However, along with the exponential growth of demand for sapphire windows, manufacturers and researchers seek to innovate the traditional SiO2 sapphire polishing method in pursuit of a greater polishing rate and higher precision.
There has been developed the Aluminum Oxide Polishing Method, which uses Al2O3 abrasive particles as the main composition of the polishing solution. Yu Jiangyong and Liu Yuling from Hebei, added nanoscale Alumina particles (with a concentration of 2%) to the SiO2 polishing solution and saw an evident increase in the polishing rate of sapphire from 9 μm/h to 11.3 μm/h, and The dilution ratio of the SiO2 polishing solution could be lowered from 1:1 to 1:2. Nevertheless, the aluminum oxide abrasives produced are often fabricated through a complicated procedure including calcination, grinding and screening, obtaining aluminum oxide with uniform and nano-scale particle sizes is difficult and expensive. The most severe flaw is that the hardness of aluminum oxide is high, the polished sapphire windows are often scratched and defective, and the viscosities of the polished surface are large, leading to washing difficulties, and it is hard to ensure the surface flatness of the polishing material. Another significant obstacle to the dissemination of the aluminum oxide polishing technique is the high requirements for the rotation speed of the equipment. Therefore, using Al2O3 as a polishing liquid is still at the experimental stage at present.
Researcher K. Bakshi and his companions from the United States utilized alpha or beta silicon carbide polished sapphire with an average particle diameter of 100-400 nm. He first softened the silicon carbide surface with a silica coating in an attempt to reduce surface scratching on the sapphire. The reason is that the silica coating can passivate sharp corners of the silicon carbide particles. The core of the particle is still silicon carbide with higher hardness, and the abrasive maintains a better polishing removal rate. He also incorporated a composite polishing solution of silicon carbide and silicon dioxide (30% SiC, 70% SiO2) to polish R-orientation sapphire, and the polishing rate can be increased 1.3-1.5 times greater than SiO2 polishing liquid without adding SiC.
Zong Simiao and Liu Yuling, from China, utilized a polishing solution self-developed in their lab with a higher pH value of about 12.5, and a high abrasive concentration (50%), under a rapid temperature rise to 45 °C. The polishing rate of sapphire can reach 11.3 μm /h. Li Shurong and Jin Zhuji from Dalian studied the effect of different pH control agents on the polishing rate, comparing four inorganic bases, including NaOH, KOH, Ca(OH)2, and Ammonia, and found that KOH can improve to a significant extent the sapphire polishing rate. Researchers also found that adding a suitable amount of Fe-Nx/C activator agents to the SiO2 polishing solution also improves the polishing rate (The Fe-Nx/C activator contains Fe2O3, Fe3O4, and nitrogen).
Apart from the polishing solution, the polishing apparatus also has a great impact on the polishing rate. Leveraging the Vibration of Ultrasonic Wave can elevate the removal rate (at about 3.8 times) in the polishing course of sapphire with SiO2 solution, and the roughness could be reduced too. Possible reasons are: (1) The motion path of silica particles on the sapphire surface is lengthened; (2) The pressure between the interface of SiO2 particles and sapphire is increased. Therefore the vibration and cavitation of ultrasonic waves strengthen the effect of silica particles on the surface of the sapphire. In practical operation, ultrasonic is applied to the polishing head of the single-sided polishing machine.
In addition, the orientation of sapphire windows is also an essential factor in the polishing rate. In general, the polishing rate of A, M, and R orientation is slower than that of C-cut sapphire. This is because the orientation index of C-cut sapphire crystal is (0001), and the atomic bonding structure is O-Al-Al-O-Al-Al-O, whilst the orientation indice of M and A cut sapphires are (10-10) and ( 11-20), their chemical bonding structure is Al-O-Al-O. The Al-Al bond is weaker than the Al-O bond, and the polishing of C-cut sapphire encounters the Al-Al bond in the main, so the polishing rate of c-cut sapphire is faster in comparison.
In real-life production workshops, the polishing precision requirement of A-cut sapphire windows is often not high. SiO2 polishing liquid and non-woven fabrics will work well. Whilst in the case of c-cut sapphire windows higher precision is often required. Polishing accessories from Universal Optical are prevalent, with the removal amount of polishing reaching 10 μm/90min.