Microstructure, roughness, and stress properties of silicon coatings for shape correction of Kirkpatrick–Baez mirrors

Abstract

Coating techniques are investigated for accurate shape correction of Kirkpatrick–Baez (KB) mirrors. Au and Pt have been used as coating materials to obtain elliptical KB mirrors from flat or cylindrical Si substrates. However, due to large differences in the thermal expansion coefficients between Au (or Pt) and Si, thermal stress can be induced not only when coating the substrate, but also during use when focusing synchrotron beams. Si is a promising alternative for profile coating because not only the same material as substrate can induce much smaller thermal stress, but also Si is generally smooth. The microstructure and roughness of 1000- and 2000-nm-thick Si coatings deposited at two different pressures (0.133 and 0.266 Pa) are explored. When the thickness increases from 50 to 2000 nm, the film microstructure and surface morphology change and root mean square roughness increases from 0.164 to 0.232 nm. The increase in Si coating thickness contributes the most to power spectral density curves when the frequency is between 1 and 20 μm − 1. No obvious changes resulting from intrinsic stress are observed among the samples when the Si coating thickness is increased from 50 to 2000 nm.