Thermal stability of anti-reflective and protective a-C:H:SiOx coating for infrared optics

[Display omitted] •Antireflective a-C:H:SiOx coatings for IR optic were obtained by PACVD.•a-C:H:SiOx coating provides good mechanical protection for Si optical elements.•a-C:H:SiOx coating increases peak mid-IR transmittance of Si by 40%.•Coating has heat resistance at the temperature to 500 °C. Poor mechanical and wear-resistant properties is the main weakness of infrared multi-layer anti-reflective optical coatings. The paper deals with anti-reflective and protective coatings based on SiOx-doped amorphous hydrogenated carbon deposited on both sides of the polished single-crystalline silicon substrates. A method of plasma-enhanced chemical vapor deposition in a mixture of argon and polyphenylmethylsiloxane vapors is used for this coating. Anti-reflective properties of obtained coatings are studied by the Fourier-transform infrared spectroscopy. It is shown how the mechanical properties, chemical composition and wettability of the a-C:H:SiOx coating depend on annealing in air. It is found that the average infrared transmission of the silicon substrates with double-sided film deposition is 87% in a 3–5 µm wavelength span, while the maximum infrared transmission is about 90%. At the same time, the films have excellent mechanical properties, heat resistance to 500 °C, and chemical resistance to a sea salt solution. The obtained anti-reflective coating has a great potential to be used as an anti-reflective and protective coating for infrared optical silicon products.