In order to improve the standard uncertainty of Avogadro constant determined by the X-ray crystal density method,an automatic scanning measurement system based on the spectroscopic ellipsometry and the two-dimensional silicon(Si) sphere scanning mechanism is proposed,which is used for determining the thickness of the oxide layer on a single crystal silicon sphere with the diameter of 93.6 mm and the mass of 1 kg approximately.By measuring the oxide layer thickness at 400 points on the standard s...