Scratch-resistant antireflective coating for mid-wave infrared band

Abstract

In the 3–5 μm mid-wave infrared (MWIR) atmospheric transparency window region, infrared detectors are widely applied in spectral imaging, remote sensing and other fields, and their rapid development has put forward higher requirements for infrared optical windows. Common mid-wave infrared optical materials such as germanium and zinc sulfide have poor mechanical properties, causing them difficult to meet various harsh environments. In this paper, a 3-layer film stack is proposed to achieve high transmission and high hardness by radio-frequency reactive magnetron sputtering with the single aluminum target. Two types of AlN/Al2O3/AlN stack with top AlN thicknesses of 50 and 100 nm are studied to realize the best optical and mechanical characteristics. The average transmission increases up to 94.0% for 3–5 μm with the double-side AlN/Al2O3/AlN stacks on the silicon substrate, while the hardness is significantly improved to ∼16.6 GPa for both types. After annealing at 500 °C and 750 °C for 6 h in a vacuum environment, the spectra of the two film stacks move towards the shorter wavelength slightly, and the hardness of both are obviously enhanced. The highest hardness of the film system reaches ∼20.8 GPa, making it far more resistant to sand erosion than ordinary antireflection films. The result shows that this film stack can withstand high temperature and gravel impact tests, meeting the service requirements of the harsh environment of the MWIR optical system.