Preparation and protection of ZnS surface sub-wavelength structure for infrared window

Zinc sulfide (ZnS) is one of the most important far-infrared optical candidate materials. However, it is indispensable for ZnS to improve its antireflection and protection especially under harsh environments. Compared with conventional surface antireflective films, surface sub-wavelength structures (SWSs) have some unique advantages. In this paper, dewetting nickel dots as etching mask on ZnS substrates were obtained by thermal annealing of as-sputtered Ni films on the intermediate layers of as-sputtered silicon films. ZnS SWSs were then prepared by inductively coupled plasma (ICP) etching. Finally, HfO ₂ films were deposited on ZnS SWSs by atomic layer deposition. The influences of SWSs and ALD-derived HfO ₂ films on the transmittance and protection of ZnS were both investigated. The experimental results show that ZnS SWSs can increase both far-infrared transmittance and surface hydrophobicity. Meanwhile, ALD-derived HfO ₂ films can provide good protection. Compared with bare ZnS, HfO ₂ -coated ZnS with double-sided SWS exhibits average transmittance of 79.2% other than initial 69.1% in 8–12 μm, better hydrophobicity with the wetting angle of 119.7°, and higher composite nano-hardness increased from 25.00 to 38.68 GPa. ALD derived HfO ₂ films on ZnS surface sub-wavelength structures have excellent application prospective for infrared optical windows.