Characterization and application of pulsed DC sputter deposited hydrogenated amorphous carbon for durable multilayer infrared optical coatings on chalcogenide glasses

Durable optical coatings using pulsed DC sputtered hydrogenated amorphous carbon (a-C:H) were deposited on chalcogenide glasses for the longwave infrared (LWIR-8-12 µm) spectral band. Carbon layers were deposited with a novel, to the best of our knowledge, multilayer, consisting of a bulk layer of high hydrogen content and a thin capping layer of low hydrogen content. This produces a layer that combines low stress with high hardness and durability and good adhesion on chalcogenide glasses. Two- and four-layer a-C:H/germanium multilayer anti-reflection coatings are demonstrated with average LWIR transmittances (reflectance) of 92.4% (2.2%) and 94.4% (1.1%), respectively. Using the novel layered hydrogenation approach during carbon deposition achieved optimal optical performance, coating stress, and hardness resulting in durable LWIR anti-reflection coatings that pass both MIL-C-48497A durability requirements and MIL-C-675C (para 4.5.9) salt spray. In addition, the antireflection coatings pass 10,000 wipes of the TS1888 para 5.4.3 sand slurry wiper test. The paper also describes stress, hardness, Young's modulus, Raman, and electron paramagnetic resonance characterization of the a-C:H material, their relevance to optimizing optical and durability performance and variation with hydrogen content.