Design and fabrication of antireflection coatings on ZnS substrate

Zinc sulphide (ZnS), a significant Infrared (IR) optical material, is applied to window, lens and dome in many optoelectronic systems where a suitable antireflection (AR) and protective coating is required to enhance the transmission. In this paper, the transmittance characteristics and deposition process considerations of AR coatings on ZnS substrate are investigated in detail. Through the theoretical analyses and real process experience of common materials such as magnesium fluoride (MgF2), silicon dioxide (SiO2) and silicon monoxide (SiO), and the deposition and environmental test of yttria (Y2O3) coating, an optimum 5-layer structure is finally employed, which consists of a bonding layer with ZnS substrate, thin Y2O3 film, and a 4-layer stack of germanium (Ge) and SiO as high and low refractive index coating materials, respectively. Then, experimental implementation of the AR design is presented, especially the deposition of Y2O3, Ge and SiO materials. The fabricated coating conforms to the tests of humidity, temperature shock, abrasion and adhesion given in the MIL-C-48497 environmental stability standards and shows an average transmittance of 96% for a given application in 3-5μm wavelength region with a peak beyond 98% at 4.2μm on 3mm thick ZnS substrate. Moreover, due to stable deposition process of Ge and SiO, better AR coating with higher spectral transmission can be easily designed and fabricated through more layers of Ge and SiO.