| 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. And it is a material that, has good chemical and physical stability, permits to be grown in quite large crystals by the use of a not much difficult technique. These physical properties have caused LN to be widely used in applications such as optical modulator, surface acoustic wave(SAW) filter, high-power ultrasonic transducer, etc, and thus it is considered as the "silicon of photonics" in the age of optoelectronic.With the development of photoelectron technology, the need for photoelectron system with high level integration and high precision is increasing day by day. There are scratches, cracks and embedding on the machined LN surface when using traditional processes such as lapping, grinding and mechanical polishing because of the soft and brittle nature of LN material. Also, the material removal rate is lower than other material’s during the machining processes due to its high toughness. In this paper, we studied the LN Chemical mechanical polishing (CMP) process experimentally based on the investigation of mechanical behavior of LN crystal, which are of significant value in improving processing efficiency and surface quality.The hardness and elastic modulus were measured by nano-indentation test, the average hardness of X-, Y-and Z-cut LN wafers are12.8,9.1and10.6GPa, and the corresponding average elastic modulus are209.6,98.3and172.2GPa. The pop-in phenomenon was observed when the normal load reached about1000μN of nano-indentation, the TEM images indicated that the pop-in phenomenon was induced by the formation of twins. According to the Hertzian theory and Oliver-Pharr theory, the calculated maximum shear stress at pop-in of X-, Y-and Z-cut LN wafers are around20.6,13.3and18.4GPa, and the corresponding stress are around25.5,21,20GPa. Besides, the plastic deformation was observed during the nano-scratch test, which provided the theoretic foundation for the nanometric machining of LN crystal.Aiming at the problem of scratches and cracks on the machined surface caused by traditional process, the CMP process for LN crystal was investigated. Three kinds of polishing pad were tested, results showed that the non-woven fabrics pad was suitable for LN wafers according to its hardness, water absorptivity and material removal rate. The polishing process parameters such as pressure and velocity, and the concentration of colloidal silica, hydrogen peroxide and citric acid in the slurry were optimized. The material removal rate can reach450nm/min and the obtained surface roughness Ra can reach0.43nm by the use of CMP process.Aiming at the problem of flatness errors and edge subsiding induced by CMP, a synthetic leather pad with high hardness and a retaining ring were applied to mechanical polishing process so as to improve the surface uniformity, and then micro scratches were quickly removed by the CMP process with non-woven fabrics pad. The effect of diamond abrasive size on mechanical polishing process was investigated, results showed that the abrasive size of3.5μm is more effective than others. The effect of CMP polishing time on flatness was investigated, which showed that the longer time was worse for the uniformity. The high quality surface with the flatness of5.3μm and the surface roughness Ra of0.38nm were obtained through mechanical polishing process combined with CMP process. |
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