Protective infrared antireflection coating based on sputtered germanium carbide
This paper describes optical, durablility and environmental performance of a germanium carbide based durable antireflection 
coating. The coating has been demonstrated on germanium and zinc selenide infra-red material however is 
applicable to other materials such as zinc sulphide. 
The material is deposited using a novel reactive closed field magnetron sputtering technique, offering significant 
advantages over conventional evaporation processes for germanium carbide such as plasma enhanced chemical vapour 
deposition. The sputtering process is "cold", making it suitable for use on a wide range of substrates. Moreover, the 
drum format provide more efficient loading for high throughput production. 
The use of the closed field and unbalanced magnetrons creates a magnetic confinement that extends the electron mean 
free path leading to high ion current densities. The combination of high current densities with ion energies in the range 
~30eV creates optimum thin film growth conditions. As a result the films are dense, spectrally stable, supersmooth and 
low stress. Films incorporate low hydrogen content resulting in minimal C-H absorption bands within critical infra-red 
passbands such as 3 to 5um and 8 to 12um. 
Tuning of germanium carbide (Ge (1-x) C x ) film refractive index from pure germanium (refractive index 4) to pure 
germanium carbide (refractive index 1.8) will be demonstrated. Use of film grading to achieve single and dual band 
anti-reflection performance will be shown. Environmental and durability levels are shown to be suitable for use in harsh 
external environments.