Kryvonogov S. Development of scientific and technological foundations of chemical-mechanical polishing of sapphire meant for optics and microelectronics

The thesis is devoted to establishment of scientific and technical regularities of the obtaining of new functional sapphire surfaces possessing high optical quality and structural perfection, to be applied in micro- and optoelectronics. Investigated are the structure of damaged surface-adjacent layer and the structural perfection of sapphire surface subjected to different kinds of treatment. The absence of polycrystalline constituent in the damaged layer and the presence of plastic deformation are established. The distribution of the crystal sizes is determined, and it is shown that the parameter of the mean-square turn of the micro-fragments may be used as a criterion of the structural perfection of the surface. Scientific foundations of chemical-mechanical polishing of sapphire are developed, and the corresponding technological conditions are optimized. Proposed is a model of the process of chemical-mechanical polishing based on the assumption that the chemical reactions proceeding on the surface of sapphire between Al2O3 and SiO2 result in the formation of amorphous layer of aluminum silicate Al2SiО5. Thereat, the reaction rate is defined by diffusion of the reacting substances into the reaction zone. Orientation anisotropy and angular dependence of the rate of chemical-mecanical polishing are established. It is shown that the polishing rate diminishes in the sequence of crystallographic planes (0001) > (10-12) > (11-20). The composition of the polishing suspension based on silicon dioxide in the form of aerosil is developed; a technique for polishing sapphire with different crystallographic orientation of the surface is proposed. Obtained are sapphire substrates with the surfaces of high structural perfection (beta = 5-5.6 ang. sec.), high optical quality (the class 20/10 according to the USA standard МIL-O-13830) and the surface roughness Ra < 0.3 nm, as well as the substrates of (0001) orientation with step-terrace structure on the surface.