Study On Single Crystal MgO Substrates Lapping And Its Damage

Single crystal MgO has excellent performances such as excellent thermal stability,optical characteristic,electrical insulativity, chemical stability and mechanical properties. It is widely used as substrates for high temperature superconductor(HTS), due to its low dielectric constant (ε=9.6), low dielectric dissipation in high frequency, similar coefficient of thermal expansion and low lattice mismatch with YBCO. It's also a kind of important optical material. Used as substrates for HTS thin film, observation window material or optical lens, the single crystal MgO must be with the purity of 99.9% or more, and with ultra flat, damage-free surface. Take the epitaxial grown HTS thin film as example, the quality of substrate surface(roughness, smoothness, microstructure, surface stress, damage and flaw, physical-chemical adsorption, impurity contamination et al) has great influences on thin film performances. So,it's important to study the processing technology of single crystal MgO surface.Lapping is a conventional process for crystal machining, which is used to acquire the ultra-flat surface of single crystal MgO substrate. However, there are many problems, such as the consistency of surface quality and the efficiency in lapping process, which limit the applications of single crystal MgO. So,it's important to deeply research the lapping process for single MgO substrates with high quality and efficiency for expanding the application of single crystal MgO.Lapping, however, as other processing methods, would inevitably bring about surface and subsurface damage, which will severely affect the surface quality and must be removed by subsequent processing. At present, to the author's best knowledgy, systematic study on the surface and subsurface damage of single crystal MgO substrates has not been reported in public and the material damage mechanism of lapping MgO single crystal has not been well understood.A series of experiments of single crystal MgO substrates lapping are carried out to solve these problems. The effects of grain dimension, lapping pressure, lapping speed and some other factors on single crystal MgO substrate quality are studied. The relationship between surface roughness material removal rate and surface damage with processing parameters are obtained, the major effect factors according to quality and efficiency are determined and then an optimized processing parameter is achieved, which has been applied successfully in manufacturing of single crystal MgO substrates in the low volume. The surface topography of single crystal MgO substrates with different processing methods and conditions, are studied by scanning electronic telescope(SEM), optical telescope and 3D surface profiler. The subsurface damage is investigated by section microscopy and step etching method, and damage depth assessment is conducted on both cross-section and subsurface layer parallel to the ground surface. The feature of composition and depth of damage with different processing conditions are obtained, and the damage pattern is also analyzed. The research results will provide worthy reference for optimizing single crystal MgO substrate lapping process and polishing process.