Investigation of natural contamination layer growth on optical substrates**Supported by the National Key R&D Program of China (2016YFA0401304), the National Natural Science Foundation of China (NSFC) (61621001, U1731242, U1732268) and the Ministry of Science and Higher Education of Russian Federation within the State assignment FSRC “Crystallography and Photonics” RAS

Abstract

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/cm3). 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/cm3 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.