ROLE OF HARD CARBON IN THE FIELD OF INFRARED COATING MATERIALS

Amorphous hydrogenated hard carbon (a-C:H) is a promising new optical coating material for passive infrared materials. It offers the combination of extreme hardness, chemical inertness, and optical transparency over a wide spectral range. The optical properties of rf-plasma deposited a-C:H coatings are compared with vacuum-evaporated infrared coatings. For many applications, a-C:H solves the problem of a mechanically and chemically resistant 8 to 12 mu m coating despite its moderate absorption in the 6 to 20 mu m range. The tunability of the refractive index between 1. 8 and 2. 2 allows single layer coatings on Si and Ge with zero reflection. State-of-the-art applications, possible future developments as well as remaining technological problems of a-C:H are discussed.