Investigation Of Chemical Mechanical Polishing For Lithium Niobate Wafer

Lithium niobate (LiNbO3, LN) crystal is a kind of nonlinear optical material with a combination of excellent electro-optic, acousto-optic, piezoelectric and ferroelectric properties. LN has to be widely used in applications such as nonlinear optics, ferroelectric, piezoelectric, lightning, and thus it is considered as the "silicon of photonics" in the age of optoelectronic.LN crystals has hardness in between soft crystal and hard brittle crystal. It has a unique mechanical properties such as temperature and mechanical anisotropy properties, low damage threshold and high brittleness, when using traditional processes such as lapping, grinding and polishing, there are scratches, cracks and embedding on the machined LN surface. LN crystals are commonly used as high-performance optics, surface roughness is needed to achieve nanometer level and can not have any defects. These requirements put forward a severe challenge for the processing of LN.This study uses CMP to solve the problems which are produced by traditional manufactur-ing. By optimizing polishing liquid formulation and process parameters to get high material removal rate and good surface quality. Combining the advantages of traditional processing and chemical mechanical polishing, when using consolidation abrasive grinding, the material removal rate can reach 1.3μm/min and flatness is 6.342μm. After grinding, choosing alumina abrasive and polyurethane polishing pad to remove the damage layer, The material removal rate can reach 420.3 nm/min and Ra is 1.727 nm. Then using chemical polishing solution to polish LN crystal,1000 mL polishing solution containing sodium hypochlorite is 30g, citric acid is 16g, silicon dioxide (diameter 20nm) is 140 g, Sodium silicate is 2g, pH is 10, Ra can obtain 0.312 nm and the material removal rate can reach 341.5 nm/min. Finally there are almost no defects on surface.By using energy spectrometer and electrochemical experiments to explain corrosion ability of different polishing liquid. When using scanning electron microscope and energy spectrum, when adding sodium hypochlorite in the Polishing liquid can improve the surface oxygen content, the surface oxygen weight percentage is increased from 31.86% to 34.51%, atomic percentage is increased from 56.56% to 66.62%, adding citric acid, the surface oxygen percentage weight is increased from 31.86% to 37.1%, atomic percentage is increased from 56.56% to 74.89%; By measuring Tafel polarization curves of different solution to calculate the corrosion voltage and corrosion current to judge the corrosion ability of different solutions. At the same time, the effect of different pH and oxidant on the corrosion of polishing liquid was studied, when the polishing liquid containing sodium hypochlorite, citric acid, and pH was 11 can improve the corrosion ability of polishing solution and obtain a higher material removal rate, which can improve the processing efficiency.