Effects of sputtering power and annealing temperature on surface roughness of gold films for high-reflectivity synchrotron radiation mirrors

Gold films deposited by direct current magnetron sputtering are used for synchrotron radiation optics. In this study, the microstructure and surface roughness of gold films were investigated for the purpose of developing high-reflectivity mirrors. The deposition process was first optimized. Films were fabricated at different sputtering powers (15, 40, 80, and 120 W) and characterized using grazing incidence X-ray reflectometry, X-ray diffraction, and atomic force microscopy. The results showed that all the films were highly textured, having a dominant Au (111) orientation, and the film deposited at 80 W had the lowest surface roughness. Subsequently, post-deposition annealing from 100 to 200℃ in a vacuum was performed on the films deposited at 80 W to investigate the effect of annealing on the microstructure and surface roughness of the films. The grain size, surface roughness, and their relationship were investigated as a function of annealing temperature. AFM and XRD results revealed that at annealing temperatures of 175 ℃ and below, microstructural change of the films was mainly manifested by the elimination of voids. At annealing temperatures higher than 175℃, grain coalescence occurred in addition to the void elimination, causing the surface roughness to increase.

Investigation of natural contamination layer growth on optical substrates

The surface contamination layer on mirrors can cause significant degradation of the optical performance, which is widely observed in applications, particularly in the fabrication of X-ray focusing telescopes. In this paper, we study the natural contamination layer arising from adsorption precipitation of hydrocarbons or other organic and water molecules in the absence of any external factor. Temporal evolution of the layer formed on super-smooth fused silica, borosilicate glass, and silicon substrates is studied by X-ray reflectometry, atomic force microscopy, and transmission electron microscopy for a one-year period after surface cleaning. The general characteristics of adhesion layer growth are established and discussed. The reconstructed dielectric constant profiles demonstrate that an increase in the adhesion layer thickness, deposited mass and density over time obeys power laws with extremely small exponents. Therefore, the adhesion layer growth is rapid immediately after surface cleaning, with a - 1 nm thick layer formed within the first day on all three substrates studied, while the layer density is low （- 1 g/cm^3）. The layer growth on the fused silica and silicon substrates became very slow in the succeeding days, with only a 1.4-1.5 nm thick layer and 1.2-1.3 g/cm^3 density after one year of storage in air. At the same time, the adhesion layer growth on the glass substrate showed unexpected acceleration about two months after cleaning, so that the layer thickness reached - 2.2 nm after one year of storage. The reason for this effect, which is connected with leaching of the glass, is discussed briefly.