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The Mainstream of Sapphire Polishing Liquid-Silica or Alumina?

wallpapers Industry 2020-07-14

Sapphire has good optical, thermal, dielectric properties and excellent mechanical properties, as well as excellent chemical stability, thermal stability and radiation resistance. It is a multi-functional crystal material with excellent comprehensive performance and is widely used in semiconductor epitaxy. Substrate materials include gallium nitride substrates, SOI substrates (SOS), optical windows, and laser matrix materials.

Because sapphire has particularly stable chemical properties and high hardness, it increases the difficulty of processing sapphire materials. Sapphire wafer processing faces many technical challenges, including polishing.

The main components of sapphire polishing liquid are abrasives, pH adjusters, surfactants, chelating agents, etc. The polishing cash is one of the most critical factors that affect the effect of CMP (Chemical Mechanical Polishing). The indicators for evaluating the performance of the polishing liquid are good fluidity, uniform dispersion, no agglomeration, precipitation, delamination and other problems within a specified time, excellent abrasive suspension performance, fast polishing rate, easy cleaning and green environmental protection.

Among them, abrasives mainly affect the mechanical role in chemical mechanical polishing. The choice of brushes is mostly considered from the three aspects of abrasive type, concentration and particle size. At present, the abrasives commonly used in CMP polishing liquids are mainly single abrasives such as diamond, alumina, and silica, as well as silica/alumina mixed abrasives and core-shell composite abrasives.

Alumina abrasive has a delicate surface, hard texture, excellent wear resistance and corrosion resistance. It is suitable for surface grinding, and polishing of a variety of materials and the most commonly used on the market is α-Al2O3 abrasive with excellent performance. The disadvantages of alumina abrasives are unfortunate, dispersibility and selectivity. The polishing fluid has a high viscosity and is not easy to clean. And because the hardness of α-Al2O3 abrasive is similar to a sapphire, it is easy to cause new damage to the surface of the workpiece during the polishing process. However, because the polishing rate of alumina abrasives is better than that of silica abrasives, the LED industry has a broad market prospect. In the future, the role of Al2O3 polishing liquids in CMP will be more critical. In recent years, the research direction of alumina polishing liquids has mainly focused on nano-abrasives: preparation, surface modification of alumina and mixed application of alumina polishing solution.

Silica abrasive grains have moderate hardness, controllable particle size, good polishing selectivity, and can be prepared into silica sols with excellent dispersion properties. They are widely used in single-crystal silicon, SiO2 dielectric layer, cadmium zinc telluride crystal, titanium alloy, nickel-based alloy, Copper, sapphire crystal and other precision polishing materials. Nano-scale silica has high surface activity and can react with sapphire (α-Al2O3) surface in solid phase chemical reaction to form a softer reaction layer. Silica has a Mohs hardness of 7, which is higher than the hardness of the reaction layer and lower than the hardness of the matrix material so that the reaction layer can be removed during the polishing process without causing new damage to the surface of the matrix.

Based on the above analysis, some researchers believe that the current mainstream of the sapphire polishing industry still uses nano-silica as the polishing liquid. Finally, the roughness (Ra, atomic force microscope test) of the polished surface is below 0.3nm. During the polishing process, the silica polishing solution reacts with the sapphire surface to form an aluminium silicate dihydrate.

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Tag: Silica