The paper introduces the principle of measurement of super-smooth surface by grazing X-ray scattering method and the experimental facility based on an improved X-ray diffraction. The scattering diagrams measured on ZERODUR

Sapphire and three wafers with different roughness are treated by first-order vector perturbation theory. The results indicate that the calculated PSD all are in a good agreement with the results obtained from Atomic force microscope (AFM). We also analyze the effects of the slit width of detector and the divergence of incidence X-ray. With other idea experimental conditions

when slit width of 0.02mm and 43" incidence divergence

errors are both lower than 2% while the range of spatial frequency is higher than 0.03μm-1. We could conclude that the error decrease quickly as reducing the slit width and incidence divergence. The PSD error also decreases quickly as increasing the spatial frequency.