At present, the main methods of polished sapphire wafer polishing technologies are mechanical polishing, chemical polishing, ion beam polishing, laser beam polishing, and so on. However, each polishing method has some shortcomings. Although mechanical polishing can achieve global planarization, it is difficult to achieve nano level polishing with roughness RMS less than 1nm; chemical polishing can achieve nano level polishing, but can not achieve global planarization; ion beam polishing and laser beam polishing can not achieve global planarization, and are still in the experimental stage.
According to the physical and chemical properties of the sapphire wafer, combined with the above characteristics of mechanical polishing and chemical polishing, using chemical mechanical polishing (CMP) technology based on chemical polishing, it is a combination technology of mechanical grinding and chemical corrosion. It realizes global planarization, ultra-smooth, and nanoscale nondestructive precision polishing with the aid of grinding effect of ultrafine particles and chemical corrosion of slurry. The basic method of CMP is to rotate the wafer in the grinding fluid relative to the polishing pad and exert a certain pressure to finish polishing with the help of mechanical friction and chemical corrosion. The platform is driven by the motor, and the chip is stuck to the disc. The disc is driven by the motor and rotates in the same direction as the platform.
A layer of polishing fluid film is formed between the polished pad and the wafer, which is used to transport particles and pressure. The polishing fluid reacts with the wafer to convert insoluble substances into soluble substances (chemical reaction process), and then remove these soluble substances from the wafer surface by mechanical friction (mechanical grinding process). The two processes promote each other to form a virtuous cycle so as to make the surface of the polished wafer is smoothed and planarized globally.
In the chemical polishing process of sapphire and polishing solution, hydrolysis reaction, neutralization reaction of acid and alkali and double decomposition reaction of salt occur.
If the chemical reaction is weak and the mechanical grinding is strong, it will lead to the slow speed of the whole polishing process, and orange peel and wire drawing will occur on the surface of the polished wafer; if the chemical reaction is strong, but the mechanical grinding effect is weak, the whole polishing speed will be slowed down because the reactant resolution is slow. The reaction product can not leave the surface in time, and the reagent cannot be contacted, which block the reaction from continuing. At the same time, the surface of the polished sapphire wafers is easy to produce corrosion pits and ripples. In order to obtain a good quality polishing wafer, it is necessary to balance the chemical reaction and mechanical action in the polishing process.
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