Optical and RF transparent protective alumina thin films

Protective alumina thin films of different thicknesses (35–95 nm) were grown by pulsed RF magnetron sputtering technique at room temperature on germanium (Ge) coated black polyimide or Kapton^® substrate which is often employed as sunshield membrane on communications satellite antennas. The thin alumina top layer provides shielding of Ge coating from degradation during storage. The alumina thin film was characterized by X-ray diffraction, field emission scanning electron microscopy and X-ray photoelectron spectroscopy (XPS) techniques to investigate microstructural and electronic characteristics. Thermo-optical properties such as IR emittance, solar absorptance, reflectance and transmittance and electrical property e.g. sheet resistance of the deposited alumina films were measured. The RF losses e.g. insertion loss and return loss were measured in both Ku band (10.5–14.5 GHz) and Ka band (27–40 GHz). Finally, to prove anti-degradation behavior of alumina thin film, water droplet spreading experiments were carried out and evaluated its chemical degradation by XPS.