Deposition and characterization of gold black coatings for thermal infrared detectors

High absorptivity and low thermal mass are two important requirements for coatings applied to thermal infrared detectors. Gold black coatings are very good candidates to ensure these characteristics in the broadband infrared spectral range. A specific deposition system was designed and built at INO in order to provide gold-black coatings for different broadband detection applications including the broadband radiometer (BBR) instrument for the European Space Agency (ESA) EarthCARE satellite. A parametric study targeting uniform optical absorptance within the spectral range from 0.2 mu m to 50 mu m was conducted. Specular reflectance lower than 10% was obtained for extended wavelength range up to 100 mu m. The coating thickness ranges typically between 20 mu m and 35 mu m, with uniformity of about +/- 3 mu m over a sample surface of 10x10 mm(2). The deposit density was typically similar to 0.3% of the bulk density of gold. To singulate the blackened infrared detector pixels, a laser micromachining process was developed. The setup exhibits a 1 mu m positioning accuracy and allows for ablation of 3 mu m to 12 mu m wide channels through the gold-black thickness, while preserving the pixel and gold-black deposit integrity.