On the properties and functionality of ultra-thin diamond related protective coatings used in optical systems

In this work we investigate the applicability of sputtered developed films belonging to diamond related materials, like amorphous carbon (a-C), carbon nitride (CN x ), and boron nitride (BN x ) films, as protective overcoatings of optical systems. Their suitability is defined by the combination of optical transparency (low absorption in the VIS energy region) with the high hardness and wear resistant character of the coating. The optical systems of our interest are electron-beam evaporated TiO 2/SiO 2 multi-layered coatings grown on polymeric substrates, which exhibit the optimum antireflective (AR) optical behavior but low wear durability and hardness. Within this research frame it was found a considerable enhancement of hardness of the AR optical coatings, which were studied by nanoindentation measurements, when an ultra-thin (∼100 Å) protective coating either of a-C, CN x or BN x is grown on top of them. In order to define which coating is the most proper one for such applications we extensively studied by spectroscopic ellipsometry the dielectric response and the optical properties of the developed films, as well as the combined reflectivity and optical response of the complete systems (AR/protective coatings).