Nanoindentation And Chemical Mechanical Polishing Of Lithium Niobate Crystals

Lithium niobate (LiNbO3, LN) crystals is a kind of nonlinear optical material with a combination of excellent electro-optic, acousto-optic and piezoelectric properties. The Curie temperature is 1210℃, and LN crystals have great spontaneous polarization. The material has a good chemical and physical stability, easy to grow in lager size. It is widely used in the field of optical waveguide, optiacal modulator, frequency doubing conversion, surface acoustic wave filter, optical integration, etc, and thus it is considered as "silicon of photonics".With the rapid development of photoelectronic technology and nonlinear optics, surface of LN wafers are required to be super smooth, no abrasives embedded, no scratches. There are scratches, cracks and embedding on the surface with traditional processes such as grinding and machining because LN crystals are of low hardness and great brittleness. Also, the material removal rate is lower. The study of LN crystals in our country is behind of US, Japanese and so on.The hardness and elastic modulus were measured by nano-indentation test. The average hardness of X-, Y-, and Z-cut of LN is 11.49,8.14,10.86 GPa, and the corresponding average elastic modulus are 201.96,159.62,202.18 GPa. The plastic deformation was observed, which provides the theory basis for ultra-precision machining. After nano-indentation test, because of lattice deformation and subsurface damage, the hardness and elastic modulus are improved. It can be a good way to increase the mechanical properties of crystal by abrasive machining.In order to solve the problem of scratches and cracks on the surface caused by traditional process, Chemical mechanical polishing process based on orthogonal experimental design for LN crystals was investigated in order to explore the effect of pH, concentration of oxidant, abrasives size, flow rate and concentration of complex agent. With the increase of silica particle size, the material removal rate and surface roughness is increasing. Along with the increase of pH value, surface roughness decreases first then increases. Under the optimum combination we got from the orthogonal experimental design, Ra is 0.642 nm. The surface roughness Ra can reach 0.6 nm and material removal rate can be more than 400 nm/min through single variable experiment.Experiments about the chemical action of oxidant and the mechanical action of the abrasives were conducted. Results show that material removal rate is low under the effect of pure chemical or mechanical action. There is interaction between them. When chemical action and mechanical action come to relative balance, the polishing effect is best.Recycle experiments of polishing slurry were conducted to solve the problem of high cost and low efficiency. The effect of polishing slurry direct reusing is poor. But we can blend the old and new slurry, adding some oxidant and complex agent. It can improve the material removal rate and lower the surface roughness.